GarageBand supported audio formats

Apple GarageBand support the following 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.