In this video tutorial, I'll walk through the steps of how to change Ableton Live's default metronome sound to your own custom audio sample. In this instance, I chose to use a fart sound. :)
Mac File Path: /Applications/Ableton Live 10 Suite.app/Contents/App-Resources/Misc/Metronome/Samples
Windows File Path: C:\ProgramData\Ableton\Live 10 Suite\Resources\Misc\Metronome
(ProgramData is a hidden folder on Windows)