Since alsa has gone from 0.5 to 0.9, the current .ebuild for mplayer (mplayer-0.90_pre2-r1.ebuild) isn't good since the default configuration file contains "-ao alsa5". The line : || use alsa && audio="alsa5" \ sould be changed to : || use alsa && audio="alsa9" \
Have a bash at 0.90_pre2-r2, and tell me if it works for you. It should select audio="alsa9" if you have /usr/lib/libasound.so.2.