Mac users, here’s how to increase/decrease tempo of an audio file with GarageBand.
1. Open Spotlight. Type “GarageBand”. Select on “GarageBand” application. Start it.
2. Create a new project in GarageBand (File -> New)
3. Drag and drop your audio file (MP3, M4A etc) in GarageBand.
4. Press Alt+Ctrl+G
5. Highlight the audio track. (Track’s color changes to purple)
6. Click on Track Editor (Scissors symbol at the bottom of GarageBand)
7. Check “Follow Tempo & Pitch” in Track Editor.
8. GarageBand displays Tempo next to the PlayHead display. Click on Tempo, and adjust it to your liking.