Convert them to AIFF files. AE has never been good with MP3s due to how it works.
Converted through iTunes and it works perfect. Thank you!
Mylenium is 100% correct. Just convert your mp3 to WAV or AIFF and use those. I know that some people uses mp3 and say that everything goes perfect but for me everything goes perfect untill it does not. So it's better to spend extra seconds on converting mp3 and be sure that it will always work in AE.
Standard Video Production Audio format is WAV, 48KHZ 16 bit Stereo PCM as a minimum.
AE does not support dolby mixes directly and any compressed audio in any format can give you problems when rendering a deliverable. AIFF is also a professional format. If you need surround or dolby or anything else you have to do your audio processing in another app and then combine your video and audio in Premiere Pro or other 3rd party app capable of sending advanced audio properties to an encoder. In AE you are basically stuck with sending stereo or mono tracks to be encoded.