You can specify that mplayer use Alsa by adding "-ao alsa1x" to the command line. If you want to make it permanent, you can edit the .mplayer/config file in your home directory and add the line "ao=alsa1x" to the bottom. This can also be handy if you want to force, for example, the null video driver to be used. You could add the line "vo=null" to the file for that. To see a valid list of drivers for your compilation of mplayer, use "-ao help" or "-vo help" and the possibilities will be listed.
