diff --git a/Core/HLE/AtracCtx.cpp b/Core/HLE/AtracCtx.cpp
index 8d5d668b19..0a9dffce8d 100644
--- a/Core/HLE/AtracCtx.cpp
+++ b/Core/HLE/AtracCtx.cpp
@@ -831,7 +831,7 @@ void Atrac::SeekToSample(int sample) {
 		const u32 start = off - track_.dataOff < backfill ? track_.dataOff : off - backfill;
 
 		for (u32 pos = start; pos < off; pos += track_.bytesPerFrame) {
-			decoder_->Decode(BufferStart() + pos, track_.bytesPerFrame, nullptr, nullptr, nullptr);
+			decoder_->Decode(BufferStart() + pos, track_.bytesPerFrame, nullptr, 2, nullptr, nullptr);
 		}
 	}
 
@@ -933,7 +933,7 @@ u32 Atrac::DecodeData(u8 *outbuf, u32 outbufPtr, u32 *SamplesNum, u32 *finish, i
 		uint8_t *indata = BufferStart() + off;
 		int bytesConsumed = 0;
 		int outBytes = 0;
-		if (!decoder_->Decode(indata, track_.bytesPerFrame, &bytesConsumed, outbuf, &outBytes)) {
+		if (!decoder_->Decode(indata, track_.bytesPerFrame, &bytesConsumed, outputChannels_, outbuf, &outBytes)) {
 			// Decode failed.
 			*SamplesNum = 0;
 			*finish = 1;
diff --git a/Core/HLE/sceAtrac.cpp b/Core/HLE/sceAtrac.cpp
index 13344f209a..192c099184 100644
--- a/Core/HLE/sceAtrac.cpp
+++ b/Core/HLE/sceAtrac.cpp
@@ -948,7 +948,7 @@ static int sceAtracLowLevelDecode(int atracID, u32 sourceAddr, u32 sourceBytesCo
 
 	int bytesConsumed = 0;
 	int bytesWritten = 0;
-	atrac->GetDecoder()->Decode(srcp, atrac->GetTrack().BytesPerFrame(), &bytesConsumed, outp, &bytesWritten);
+	atrac->GetDecoder()->Decode(srcp, atrac->GetTrack().BytesPerFrame(), &bytesConsumed, 2, outp, &bytesWritten);
 	*srcConsumed = bytesConsumed;
 	*outWritten = bytesWritten;
 
diff --git a/Core/HLE/sceAudiocodec.cpp b/Core/HLE/sceAudiocodec.cpp
index 9934aa1d56..7361468063 100644
--- a/Core/HLE/sceAudiocodec.cpp
+++ b/Core/HLE/sceAudiocodec.cpp
@@ -119,7 +119,7 @@ static int sceAudiocodecDecode(u32 ctxPtr, int codec) {
 			auto ctx = PSPPointer<AudioCodecContext>::Create(ctxPtr);  // On stack, no need to allocate.
 			// Decode audio
 			int inDataConsumed = 0;
-			decoder->Decode(Memory::GetPointer(ctx->inDataPtr), ctx->inDataSize, &inDataConsumed, Memory::GetPointerWrite(ctx->outDataPtr), &outbytes);
+			decoder->Decode(Memory::GetPointer(ctx->inDataPtr), ctx->inDataSize, &inDataConsumed, 2, Memory::GetPointerWrite(ctx->outDataPtr), &outbytes);
 		}
 		DEBUG_LOG(ME, "sceAudiocodecDec(%08x, %i (%s))", ctxPtr, codec, GetCodecName(codec));
 		return 0;
diff --git a/Core/HLE/sceMp3.cpp b/Core/HLE/sceMp3.cpp
index cdd8f09964..1b337e800f 100644
--- a/Core/HLE/sceMp3.cpp
+++ b/Core/HLE/sceMp3.cpp
@@ -744,7 +744,7 @@ static u32 sceMp3LowLevelDecode(u32 mp3, u32 sourceAddr, u32 sourceBytesConsumed
 	
 	int outpcmbytes = 0;
 	int inbytesConsumed = 0;
-	ctx->decoder->Decode(inbuff, 4096, &inbytesConsumed, outbuff, &outpcmbytes);
+	ctx->decoder->Decode(inbuff, 4096, &inbytesConsumed, 2, outbuff, &outpcmbytes);
 	NotifyMemInfo(MemBlockFlags::WRITE, samplesAddr, outpcmbytes, "Mp3LowLevelDecode");
 	
 	Memory::Write_U32(inbytesConsumed, sourceBytesConsumedAddr);
diff --git a/Core/HW/Atrac3Standalone.cpp b/Core/HW/Atrac3Standalone.cpp
index b109effd44..63845bbc37 100644
--- a/Core/HW/Atrac3Standalone.cpp
+++ b/Core/HW/Atrac3Standalone.cpp
@@ -15,7 +15,8 @@ inline int16_t clamp16(float f) {
 // Test case for ATRAC3: Mega Man Maverick Hunter X, PSP menu sound
 class Atrac3Audio : public AudioDecoder {
 public:
-	Atrac3Audio(PSPAudioType audioType, int channels, size_t blockAlign, const uint8_t *extraData, size_t extraDataSize) : audioType_(audioType) {
+	Atrac3Audio(PSPAudioType audioType, int channels, size_t blockAlign, const uint8_t *extraData, size_t extraDataSize)
+		: audioType_(audioType), channels_(channels) {
 		blockAlign_ = (int)blockAlign;
 		if (audioType == PSP_CODEC_AT3PLUS) {
 			at3pCtx_ = atrac3p_alloc(channels, &blockAlign_);
@@ -55,7 +56,7 @@ public:
 		}
 	}
 
-	bool Decode(const uint8_t *inbuf, int inbytes, int *inbytesConsumed, uint8_t *outbuf, int *outbytes) override {
+	bool Decode(const uint8_t *inbuf, int inbytes, int *inbytesConsumed, int outputChannels, uint8_t *outbuf, int *outbytes) override {
 		if (!codecOpen_) {
 			_dbg_assert_(false);
 		}
@@ -84,12 +85,20 @@ public:
 				*outbytes = nb_samples * 2 * 2;
 			}
 			if (outbuf) {
-				// Convert frame to outbuf. TODO: Very SIMDable, though hardly hot.
-				for (int channel = 0; channel < 2; channel++) {
-					int16_t *output = (int16_t *)outbuf;
+				_dbg_assert_(outputChannels == 1 || outputChannels == 2);
+				int16_t *output = (int16_t *)outbuf;
+				const float *left = buffers_[0];
+				if (outputChannels == 2) {
+					// Stereo output, standard.
+					const float *right = channels_ == 2 ? buffers_[1] : buffers_[0];
 					for (int i = 0; i < nb_samples; i++) {
-						output[i * 2] = clamp16(buffers_[0][i]);
-						output[i * 2 + 1] = clamp16(buffers_[1][i]);
+						output[i * 2] = clamp16(left[i]);
+						output[i * 2 + 1] = clamp16(right[i]);
+					}
+				} else {
+					// Mono output, just take the left channel.
+					for (int i = 0; i < nb_samples; i++) {
+						output[i] = clamp16(left[i]);
 					}
 				}
 			}
@@ -113,6 +122,7 @@ private:
 	ATRAC3PContext *at3pCtx_ = nullptr;
 	ATRAC3Context *at3Ctx_ = nullptr;
 
+	int channels_ = 0;
 	int blockAlign_ = 0;
 
 	int outSamples_ = 0;
diff --git a/Core/HW/MediaEngine.cpp b/Core/HW/MediaEngine.cpp
index cd5d1b9b1e..c7f89de5c4 100644
--- a/Core/HW/MediaEngine.cpp
+++ b/Core/HW/MediaEngine.cpp
@@ -1078,7 +1078,7 @@ int MediaEngine::getAudioSamples(u32 bufferPtr) {
 		}
 
 		int inbytesConsumed = 0;
-		if (!m_audioContext->Decode(audioFrame, frameSize, &inbytesConsumed, buffer, &outbytes)) {
+		if (!m_audioContext->Decode(audioFrame, frameSize, &inbytesConsumed, 2, buffer, &outbytes)) {
 			ERROR_LOG(ME, "Audio (%s) decode failed during video playback", GetCodecName(m_audioType));
 		}
 
diff --git a/Core/HW/SimpleAudioDec.cpp b/Core/HW/SimpleAudioDec.cpp
index abf7722d1d..2b7a16055f 100644
--- a/Core/HW/SimpleAudioDec.cpp
+++ b/Core/HW/SimpleAudioDec.cpp
@@ -67,7 +67,9 @@ public:
 	}
 	~MiniMp3Audio() {}
 
-	bool Decode(const uint8_t* inbuf, int inbytes, int *inbytesConsumed, uint8_t *outbuf, int *outbytes) override {
+	bool Decode(const uint8_t* inbuf, int inbytes, int *inbytesConsumed, int outputChannels, uint8_t *outbuf, int *outbytes) override {
+		_dbg_assert_(outputChannels == 2);
+
 		mp3dec_frame_info_t info{};
 		int samplesWritten = mp3dec_decode_frame(&mp3_, inbuf, inbytes, (mp3d_sample_t *)outbuf, &info);
 		*inbytesConsumed = info.frame_bytes;
@@ -101,7 +103,7 @@ public:
 	FFmpegAudioDecoder(PSPAudioType audioType, int sampleRateHz = 44100, int channels = 2);
 	~FFmpegAudioDecoder();
 
-	bool Decode(const uint8_t* inbuf, int inbytes, int *inbytesConsumed, uint8_t *outbuf, int *outbytes) override;
+	bool Decode(const uint8_t* inbuf, int inbytes, int *inbytesConsumed, int outputChannels, uint8_t *outbuf, int *outbytes) override;
 	bool IsOK() const override {
 #ifdef USE_FFMPEG
 		return codec_ != 0;
@@ -264,7 +266,7 @@ FFmpegAudioDecoder::~FFmpegAudioDecoder() {
 }
 
 // Decodes a single input frame.
-bool FFmpegAudioDecoder::Decode(const uint8_t *inbuf, int inbytes, int *inbytesConsumed, uint8_t *outbuf, int *outbytes) {
+bool FFmpegAudioDecoder::Decode(const uint8_t *inbuf, int inbytes, int *inbytesConsumed, int outputChannels, uint8_t *outbuf, int *outbytes) {
 #ifdef USE_FFMPEG
 	if (!codecOpen_) {
 		OpenCodec(inbytes);
@@ -315,6 +317,7 @@ bool FFmpegAudioDecoder::Decode(const uint8_t *inbuf, int inbytes, int *inbytesC
 
 	if (got_frame) {
 		// Initializing the sample rate convert. We will use it to convert float output into int.
+		_dbg_assert_(outputChannels == 2);
 		int64_t wanted_channel_layout = AV_CH_LAYOUT_STEREO; // we want stereo output layout
 		int64_t dec_channel_layout = frame_->channel_layout; // decoded channel layout
 
@@ -436,7 +439,7 @@ u32 AuCtx::AuDecode(u32 pcmAddr) {
 			nextSync = (int)FindNextMp3Sync();
 		}
 		int inbytesConsumed = 0;
-		decoder->Decode(&sourcebuff[nextSync], (int)sourcebuff.size() - nextSync, &inbytesConsumed, outbuf, &outpcmbufsize);
+		decoder->Decode(&sourcebuff[nextSync], (int)sourcebuff.size() - nextSync, &inbytesConsumed, 2, outbuf, &outpcmbufsize);
 
 		if (outpcmbufsize == 0) {
 			// Nothing was output, hopefully we're at the end of the stream.
diff --git a/Core/HW/SimpleAudioDec.h b/Core/HW/SimpleAudioDec.h
index 904b8f927f..ddf3d9c589 100644
--- a/Core/HW/SimpleAudioDec.h
+++ b/Core/HW/SimpleAudioDec.h
@@ -37,7 +37,7 @@ public:
 	virtual PSPAudioType GetAudioType() const = 0;
 
 	// inbytesConsumed can include skipping metadata.
-	virtual bool Decode(const uint8_t *inbuf, int inbytes, int *inbytesConsumed, uint8_t *outbuf, int *outbytes) = 0;
+	virtual bool Decode(const uint8_t *inbuf, int inbytes, int *inbytesConsumed, int outputChannels, uint8_t *outbuf, int *outbytes) = 0;
 	virtual bool IsOK() const = 0;
 
 	// These two are only ever called after Decode, so can initialize on first.
diff --git a/UI/BackgroundAudio.cpp b/UI/BackgroundAudio.cpp
index 0231c77edc..2f6c7ea3f7 100644
--- a/UI/BackgroundAudio.cpp
+++ b/UI/BackgroundAudio.cpp
@@ -219,7 +219,7 @@ public:
 		while (bgQueue.size() < (size_t)(len * 2)) {
 			int outBytes = 0;
 			int inbytesConsumed = 0;
-			decoder_->Decode(wave_.raw_data + raw_offset_, wave_.raw_bytes_per_frame, &inbytesConsumed, (uint8_t *)buffer_, &outBytes);
+			decoder_->Decode(wave_.raw_data + raw_offset_, wave_.raw_bytes_per_frame, &inbytesConsumed, 2, (uint8_t *)buffer_, &outBytes);
 			if (!outBytes)
 				return false;