Volume gain multipler for custom dialog in Sound Module

[Xeletron]

Contractor dialog for custom heists is notoriously bad, its usually super quiet for the creators, so they have to amplify them to the point where they become distorted and awfull to listen to.

Even when re-adding lines ripped straight from the game they sound much lower quality and less audible compared to vanilla lines.
Of course the game normally lowers music volume while a dialog line is playing, but even when this feature is replicated the lines still weirdly feel quieter.

A solution that could be investigated is allowing to set a gain multiplier for sounds in the Sound Module, which would add that multiplers to the _compute_gains function when XAudio retrieves the desired volume. This way custom voicelines could always play louder than normal sfx and creators could individualy set the multiplier to fit their own lines (within a reasonable scale of course)

Unfortunetly with XAudio's inconsistent volume levels between users this flat multiplier could still cause problems, so it would need thorough testing to come up with the right solution