LAV can also do downmixing if you enable it in the Mixing config tab.

Then MPC's renderer outputs the audio directly to the driver because you have enabled exclusive mode, and so it downmixes (matrixes) it to 5.1 as your audio device says that's what it supports. In your setup, LAV is decoding and outputting the audio stream to the renderer as-is, which is why you see '8 channels' output and the 8 green bars moving during playback. You have to differentiate the audio decoder (LAV) and renderer (MPC's internal one), which are two separate 'steps' in the playback chain.

On a related note, would it be possible to integrate an option into MPC-HC so one can manually set the number of output channels, so one could bypass that windows api thingy? The only thing I can theoretically go by is that on the same stats page in lav audio 8 green bars are reacting when playing back an 8-channel media. But what is it outputting in the end? At the same time, 'internal audio renderer' says it's downmixing from 8 to 6, and 'lav audio decoder' says it's input and output is 8 channels.

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