Here's what I get doing that as the nobody user:

# sudo -u nobody ls -l /usr/bin | grep mplayer
-rwxr-xr-x 1 root root 8207616 Dec 4 2008 gmplayer
-rwxr-xr-x 1 root root 7741080 Dec 4 2008 mplayer


The mplayer binary should have world execute permissions. I've tried putting 777 permissions on all the directories involved in the process but nothing has worked.

I'm starting to think that maybe this is a PHP issue. I feel like I've seen this before, where PHP didn't have permission to run a shell script. I have all the required entries in php.ini, but the default php.ini file had a lot of other directives.