Listen room Insulation only

It will definitely reduce the sound reflections you’d get if the walls were sheetrocked. It might result in a 'deader' space.

going to try it and report

Do youself a favor and cover it with polyethylene. This will keep the small particles from escaping and possibly causing resperatory issues. 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. 
I am a builder and always noticed a tickle in my throat until the insulation was covered

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.