Thanks for uploading that file.
It's not easy to tell what the correct a/v sync should be on that video, but comparing mpegplayer with "mplayer" (the video player I use for reference), there is definitely a large difference, so something is wrong with mpegplayer.
Before the AV sync commits, Rockbox would just play the video and audio at the correct speeds without taking into account the timestamps in the file. mpegplayer now attempts to play the video and audio according to those timestamps, which is where it must be going wrong in your file.
If you (or anyone else) have other videos with similar problems, they would be useful to have as well.