I had the same issue. If it's in your music folder, try using ;
Pause Tag "your music"
Resume Tag "your music"
Set Silent / Set not Silent for all sound effects (put in sound folder).
Put background music in music folder so it streams.
Set Silent / Set not Silent doesn't work on music folder on IPhone.
Probably something to do with how it streams when in music folder.
You should read
https://www.scirra.com/manual/109/audio