Many FLV files when played with MPlayer do not have their audio and video synchronized. For example, the person's lips move and then their voice is heard. You cannot jump to any particular point in the video by pressing the arrow keys or clicking at that particular point in the playbar.

If you press the forward ( --> ) key when the sound is desynchronized, the audio and video will synchronize temporarily (for a few seconds) and then the lag appears once more, instead of moving forward in the video for 10 seconds. If you play the video from the command line, you can see that the "A-V" tag is more than one, and keeps increasing or decreasing slowly.

Is this a bug in Mplayer or a particular codec? I faced the same problem in RH 9. There is nothing wrong with the files as they play perfectly well when incorporated in a swf file or played with the stand alone Windows FLVPlayer.

What do I do?

Have you tried different outputs for sound and video?
mplayer -vo help
mplayer -ao help

I executed the commands, but is this really problem related to the output drivers? Some FLV files play properly while some don't, yet both of them work properly when used in Windows FLVPlayer or when they're incorporated in swf files.