MP4 is a container, which, according to mp4 standards, can encompass one of two codecs: Mp3 or AAC. AAC is the successor to the mp3 standard. AAC has the best sound-per-byte quality, and is not proprietary.
Don't use ogg; almost nothing supports it in the real world (devices/hardware).
Don't use FLAC; you don't need it.
Ogg and flac are specialty codecs made for a specific type of purpose, neither of which is your purpose. Go with AAC. Some mp3 players support AAC. Oh, and by the way, i still recommend using MP3 over aac for portability reasons. Slipheed has a good point also about not converting them for reasons that they are already lossy.
There is a sound converter that you can use to convert entire directories all at once. Try:
yum -y install soundconverter
And while we are at it: h264 video is the successor to divx/xvid; so that is the video codec you would want to use if you ever do video projects.
Last edited by Vector; 30th August 2009 at 04:18 AM.