I have actually been wondering this for quite some time. With Premiere Pro, what audio file formats can you import? I'm not so fast assuming .mp4 or any iTunes formats, but even if then, WAHOO! But just wondering...
I'm not sure if this applies to all formats (in fact, I'm pretty sure it doesn't apply to some of them), but I know that with some formats, as long as you have the codec installed, Premiere can import it. I have noticed this most specifically with .avi variants.
Premiere is good about importing different file types with different sampling rates, but as a rule, avoid doing so. I have always had the best result with any NLE when all the audio files are the same. Same type, same bit rate, same sampling rate. That way, your NLE doe not have to decode and interpret many different files "on the fly".
Make every file you want an uncompressed .wav, 16 bit 44.1k or 48k sampling rate. You will be glad you in the long run.