Fix volume scaling for mono inputs, etc.

Fixes #4423.

Core/HW/SimpleAT3Dec.cpp

@@ -142,7 +142,8 @@ bool SimpleAT3::Decode(void* inbuf, int inbytes, uint8_t *outbuf, int *outbytes)
 			ERROR_LOG(ME, "swr_convert: Error while converting %d", swrRet);
 			return false;
 		}
-		__AdjustBGMVolume((s16 *)outbuf, numSamples * frame_->channels);
+		// We always convert to stereo.
+		__AdjustBGMVolume((s16 *)outbuf, numSamples * 2);
 	}
 
 	return true;