AE's Prefs has a setting to only Preview audio when the Preview engine is able to preview audio in realtime. See the yellow area in the image below.
Lastly, most pros I know do not use MP3 files in AE. MP3 files not ideal files to use for production. I suggest you convert the MP3 to WAV, or AIFF; essentially an uncompressed audio format. Adobe Audition does this well.