HOW TO CHANGE MANUALLY THE NATIVE SOUND OF THE RENOISE METRONOME
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1.- Go to the "Skin" folder of Renoise ("C:\Program Files\Renoise X.X.X\Resources\Skin" or equivalent) 
2.- Copy into this folder all the Metronome_X.wav files.
3.- Replace the name Metronome.wav of the native wav file with Metronome_0.wav
4.- Now rename the desired file Metronome_X.wav to Metronome.wav.
5.- Restart Renoise. Now the metronome will sound with the new sound!