What determines which volume control is used by default?

[Michel Salim]

I'm asking this, because on my Thinkpad T61 running Rawhide, the mixer
applet by default controls the microphone input volume, rather than
PCM.

That can be fixed easily. Harder to fix is which volume is adjusted
when the keys set in Keyboard Shortcuts to control volume are pressed.
They still adjust the microphone volume!

I'm guessing the problem is a bit deeper than at the GNOME level.
Somewhere in the ALSA stack, perhaps?

Thanks,