Force downmix to mono on Linux
Solution 1:
I haven't done any extensive testing, but this ~/.asoundrc file (or /etc/asound.conf) should work for anything that uses ALSA. Assumes that you only want to deal with two in-channels (left and right), and that your soundcard is at hw:0.
pcm.!default makemono
pcm.makemono {
type route
slave.pcm "hw:0"
ttable {
0.0 1 # in-channel 0, out-channel 0, 100% volume
1.0 1 # in-channel 1, out-channel 0, 100% volume
}
}
Check out the official documentation for details on how to fine-tune/complicate things further.