I am running a Macbook Pro 13" icore 7 with mountain lion 10.8.2 and iTunes plays audio only on mp4s and Safari and Firefox won't play videos. Youtube shows the adverts and then stops playing the main video even though it continues to load the video. I have done several clean installs from the restore mode and then migrated my user files to the new install. Quicktime plays all videos in the iTunes library perfectly. Is this a user preferences, cache, or OS X system problem? Latest flash player is installed.