I've been watching more and more videos through MythVideo these days, and have been frustrated with audio/video lag. I messed around with the settings and finally came up with something that seems to be working very well. Here's my new mplayer command for MythVideo:
mplayer -framedrop -monitoraspect 16:9 -fs -zoom -quiet -vo xv -ao oss %s
Switching to OSS seems to have fixed the problem.