If you drag an uncompressed audio file from the Finder or from another application into the timeline of a GarageBand project, it might play back at the wrong speed.
This happens when you use audio files at sample rates other than 44.1 kHz. Files with higher sample rates, such as 48 kHz, will play back too slow. Files with lower sample rates will play back too fast. You should only use files with the supported sample rate in GarageBand projects.