Are you using the LW Player ?? It is highly recommended that you use an alternitive Media Player as the LW Player is nothing but JUNK. See Link below.
Disable the LW Player and make VLC Media Player the Default Player
CODECs list for Movies/Audio/ Alternate players / Video help forums/ video-mp3 fixers
Within that thread is a link to download VLC player...it's free & has codecs installed to play most file types. It's a very reliable player