Any sound for now will do, just use placeholders, I can find appropriate sounds.

The sounds are in the react-pro-metronome library (in source). I'm not sure why they are mp3, aac, ogg and wav for each sound, I assume for browser compatibility.

I got sounds from freesounds.com or something similar to this and used online converters to get multiples.

Extra sounds would be changed in the menu.
Perhaps 3 dropdown menus.
Change sound for:
1st beat sound[ sound 1, sound 2, sound 3]
standard beat sound [ soundX, soundY, soundZ]
fill highlight sound [ping1, ping2, ping3]