GarageBand supported audio formats

Recording
GarageBand supports recording at 16 bit/44.1 kHz.

Other  formats, such as 48 kHz audio, are not supported for recording.  GarageBand will change the format for the selected audio device if  possible.

Importing
You can import AIFF and WAV files  as long as they are 16 bit. If you import an MP3 file, it will be  converted and stored in your project as an AIFF file.