Listen room Insulation only

Addendum

IF the paper backing is stapled over the studs is fastened over the face of the stud rather than inside it, you may be alright w/o the polyethylene.

should read

IF the paper backing of the insulation is fastened over the face of the stud rather than inside it, you may be alright w/o the polyethylene.

Nothing you can do will make a garage sound good.Good luck though.

Is it a heated garage?Does it stand alone?  If it has a common wall with the house you don't want another vapor barrier with a wall that already has one. This would create moister problems. Need more details!

This vapor barrier has been a source of controversy for 50+ yrs or more. If you buy into the idea, then use Tyvek. It may be cheaper. My whole point is that you do not want to be breathing the dust from fiberglass  insulation for long periods of time. It needs to be covered for habitation

Don't use fiberglass.  Roxul is awesome stuff.  I totally agree with artemus and cover the walls with Tyvek.  Not only will it stop fibres, it will keep everything cleaner and look finished.  
You do realize the sound will really bleed outside...?