Try converting your MP3 to an uncompressed format (like WAV or AIF) before you import it into AE.
If that doesn't work, post back here but tell us more about the file specifications, and the specs of your computer.
I have tried that but it still plays twice as fast.
File specs Time code default base 30.
8 bits per channel
sample rate 48.000 kHz
3Ghz intel core i7
8GB 1600mgz dd3
intel hd graphics 4000 768mb
Try checking the audio sample rate for your AE project (file/project settings) matches the audio sample rate
8 bit audio is very unusual since about 1995 - where did it come from? The problem is almost certainly that AE is processing the audio as 16 bit. You can go into AE Preferences and switch AE's audio prefs to 8 bit, but it would be much more practical to convert your mp3 file to a lossless 16 bit 48khz file.