I have VLC v1.1.13 player installed on CentOS 6.2 64 bit and for the most part it seems to be working fine. However, some but not all .mp4 video files exhibit one of two issues. No sound or broken up sound. I have some .mp4 files which play just fine. I believe all of them were downloaded from youtube for whatever difference that makes(?) Totem media player will not play .mp4 at all. In fact, after installing VLC I removed Totem as I have always used VLC.
I just built a CentOS virtual machine and tried playing one of the offending videos. Totem would not play it and wanted to install some codecs. I allowed it to do so and it downloaded half the Internet. After all this it says "Could not decode stream". I installed VLC and it will play the video but with no sound.
The offending .mp4 videos play fine in VLC 1.1.11 on Windows XP.
Obviously I am missing some required component. Just as obviously I have no clue as to what I need nor where to look. Past experience with VLC has shown that it generally installs everything it needs to play just about anything known to man.
I have looked at the CentOS wiki on setting up multimedia on CentOS 5. I believe my experiment on the virtual machine installed most of the packages shown there. I will track them down one by one and report back if that provides any information.