[invader-build] Some values in sound tags are incorrectly defaulted.

[Aerocatia]

Current behaviour is to set both zero_gain_modifier and one_gain_modifier to 1 if both are 0. This is incorrect, the set defaults are variable based on the configured sound class. Some examples are that game event will default them to 1, 1, ambient nature will default to 0, 1, and ambient machinery will default to 0.5, 0.5 respectively. Every possible class will need to be tested.

[SnowyMouse]

Oh wow. This will be fun!

[SnowyMouse]

Fixed in 0e73afa

Basically, some sound classes do NOT set the zero gain modifier to 0, only the one gain modifier.