are you using only mplayer or gmplayer? (the gui version of mplayer)
if your using just mplayer, try adding those alsa lines to either the system wide config file '/etc/mplayer/mplayer.conf' or the specific user config file '~/.mplayer/config'
if your using gmplayer then edit the users config file '~/.mplayer/gui.conf'
I use gmplayer because its easier. the file i mentioned in my first post was for gmplayer, but it just occured to me that you may not be using it
you should be able to find out the main channel in alsa mixer by lowering/increasing each channels level and seeing which one does that, then replace the 'Analog Front' with whatever the name of the channel is. you'll have to experiment with this, make sure you've got an mp3 playing in your media player while doing it.