Myth seems to work fine, as the input is only 2 channels. When I watch a DVD or something from mplayer, the mplayer volume seems to control the front and rear speakers, but not the center. This is extremely annoying when I need to pump up the volume to understand speech. It only gets worse when I turn the volume up, because then there's more junk over the voice channel.

Is there some way to get mplayer to control all channels equally?

Bump

All I need to know is what mixer do I need to specify on the command line. It wants to control the pcm mixer right now, but I want it to control the master.

Does anybody know?

What you could try is add "-ao alsa" (without the quotes) in the mplayer command line (see man mplayer) and have alsamixer open in a (SSH) shell to play with the several slides while playing some content.