I use rexrainbow 's plugin:
I'm guessing you've already tried it since you mentioned that you can't set the volume in db, but you actually can. Just use "0db" (with the apostrophes) instead of 1, for instance.
In case you absolutely NEED some kind of code without a plugin, check this out:
I've used this before and it works great but it's really finnicky in some aspects. For instance I had this code so the music would change when the player entered a door and appeared somewhere else, but if he went back quickly before the last fade was over, the music wouldn't change.
Rex's plugin works great for me though, even on Android. Still haven't tested it on iOS though.