Well, It depends waht graphical enviroment do you use?
If you use custom de, you can add mixer .... to ~/.xinitrc and ~/.xsession or link .xinitrc to .xsession and viseversa.... it depends which one of them is executed....
I link them, but, since I use fvwm, I specify, that mixer should set default values on fvwm startup
EDIT
s/like/link/
s/sed/set/