Search BGM Script in buildings more quietly

sorry for my bad English there I from Germany come and my English is not so good.

Aim looking for a Script.

if we take sometimes in me I am in a village, town or a kingdom.
There runs a bgm sound and if I go in buildings the music should become quieter about approx. 20%.

Is there something so or is it also differently possible?

You don't need a script for this. You can manually just select to turn the music down the same way that you select what music to have on the map in the first place. If it's the exact same song, then you won't even have to worry about stopping the track first, because it'll keep going exactly where it had been before but with the new volume level instead.