There are about a zillion, well maybe not a zillion but there are a lot of Quicktime Codecs. Audio, but default should be good if your original is good. If the original audio is an MP3 and you are using an earlier version of AE you must convert the original audio to something elce and replace it.
More info about your system and your settings would help a bunch.
And to add one more vote: You are telling us exactly - erm - zero. We do not even know what version of AE, much less on what operasting system and then all the annoying little details like the source of the files, your comp settings, render settings and whatnot. Pardon my candor, but that's a typical "My car doesn't run." post that's utterly useless.