I am insulating my theater walls in a below grade basement in Minnesota. How much insulation is needed and does it need to fill the complete space to the DW for sound proofing? Exterior wall is 2X4 over cinder block. Interior walls staggered stud construction. Code calls for r-11 (3.5") with 4 mill plastic VB on exterior walls. No restriction on interior walls. Clips on studs then hat channel leaves some free space to the double DW. Should I add extra fiberglass below channel over the VB? Another option is to overfill the cavity with 6" fiberglass add VB but allow it to balloon out. Third option leave off the VB (its been inspected) and allow the extra fiberglass to billow out to the DW. Moisture was a problem in the past, but I have water proofed the walls with no moisture now for 2 years. Any suggestions?

Do not over fill any cavities. You just need r-11/r-13. Fiberglass should not be compressed or else you get diminishing soundproofing returns.

2x4 = r13

2x6 = r19

Some would advise you to leave the exterior perimeter walls with a 1 inch air gap to provide some room for air to circulate and dry out if you have any future moisture issues including condensation in the summer.