mplayer started too loud compared to XMMS (actually too loud period - I almost always first opened alsamixer to bring down the volume if I wanted to run mplayer, which was getting annoying as well). One line in ~/.mplayer/config:
volume=15
While I'm on the topic, lets save the other options that I found useful in the config file. See the man file if sth is not selfevident.
softvol=1
subfont-text-scale=2.5
vf=screenshot,eq,hue
unicode=yes
utf8=yes
idx=yes
ao=alsa
subfont-encoding=unicode
sub-fuzziness=1
volume=15
