The volume parameter for the custom sounds seems to only be effective after the engine has reached the point where volume should start increasing. Then, if you bring it back down to idle, it works properly, but if you shut off the engine and start it again the sounds all behave like they're volume-adjustment-less.

Turns out this is easy enough to fix. Will do this in the next release.

Fixed in V19.2.0.

I'm going to let @BlazeWolf1122 handle this one, as he's the one who put that system in.