See this chapter from the manual:
Local variables are often used when you need a "temporary" variable in an event for some calculations. Note, that local variable is reset on the next tick, so if you use "Wait" in the event, the variable will be reset after it.
If you need the variable to not reset, you can set it as Static. For example, you can create an event group Audio, add local static variable MusicVolume there. This variable will only be accessible inside the Audio group (which is good, because you don't need it anywhere else) and it will not be reset even if you reset all global variables.