You can put the following in your rc.local
/usr/sbin/aumix -d /dev/mixer1 -v100 -t60 -b60
(check out the man page for better options--also try it from the command line first to make sure it works
or set your mixer the way you want then from cli (as root):
/usr/sbin/alsactl store
then put in your rc.local:
/usr/sbin/alsactl restore
You will need to try these from the command line before putting them in your rc.local--I'm on a winders box right now so i'm running off memory.
Happy slacking
dan
PS--make sure your mixer really is /dev/mixer
1 it may be 0....