If you are using gnome-mplayer and have pulse audio installed on your machine, you may notice situations where the volume is set to 100%. This appears to be an issue with mplayer and pulse audio. My suggested work around is to use the pulse audio alsa emulation. To do this select 'alsa' as the default audio output in gnome-mplayer.
The problem stems from the change in pulse audio from separate volumes per each application to a single flat volume control. So if flat volumes are disabled on your system, you will not experience this issue.
This issue happens by default on Fedora 12 when used with mplayer from rpmfusion, so adjust your configurations as needed.
RPMFusion bug #1011