How to remove the sound effect?

Go to Settings > Mod settings > Map > Red Button
Set Volume and Delay both to 0

How to use a different sound effect?

Extract the contents of the mod zip file. 
Replace the audio file located at self-destruct-button_x.x.x/audio/boom.ogg
The new file must be an .ogg and must be called boom.ogg
You will likely need to adjust the delay of the explosion to match the timing of the new sound.
Go to Settings > Mod settings > Map > Red Button > Delay