I have a lot of 5.1 flac files. They play just fine, apart from not every channel will be downmixed at the right volume to the two speakers in my headphones while playing. This is the same no matter what player I use (vlc, ffplay, mpv, parole)
The front two, centre and LFE tracks are downmixed, and I hear them, but the rear two are very quiet.
A quick speaker-test shows this to be true.
How can I fix this? So far alsamixer has been totally unhelpful, as it never seems to display each virtual speaker, just a master volume, PCM, etc.
EDIT: I installed paman, and it shows how the sound goes from the sink input as "s16le 6ch 48000Hz" and somehow becomes "s16le 2ch 48000Hz", but during that process the channels are butchered and mixed wrong. What kind of malformed wizardry is this? Where can I find a mixer that will fix this?