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Customizing built-in sounds is on our backlog.
You can change the /resources/sounds/bicycle_bell.wav file in the Bookmap.jar with any suitable wave file named bicycle_bell.wav
To do that copy your customized bicycle_bell.wav file, change Bookmap.jar extension from .jar to .zip, open it with a file manager and paste the file into the /resources/sounds folder. Rename Bookmap.zip back to .jar
You need to do it any time you update/reinstall Bookmap.
Edited: Bookmap.jar is in your Program Files/Bookmap folder (for Windows) or similar for other OS.