basement renovation - ideas?

There is new acoustical drywall available. I don't know the details, but a Google search should do the trick. It is supposed to provide good acoustic insulation and be deader sounding than regular drywall.

Think of drywall as another bass panel...it shakes.
If the room is mainly for sound, consider studs every 12 inches instead of 16", screw and glue 1/2" plywood up first, then put drywall over that with screws and glue. Even using double sheets of drywall will help.
Secondly, decouple the walls from the ceiling above. Putting in a false ceiling will both prevent you from hearing above as well as you shaking the upstairs too badly. You asked, this is an expensive way to do it.
Last suggestion, run all the extra wire you may ever want first. I always pull an extra run of Cat-5 to every outlet just for future use. The cost of the wire is minimal vs having an electrician later.

Double drywall with 1 layer 1/2" AND THE OTHER 5/8" Don't do the same thickness for both layers (diff res. freq.)The green glue sandwiched between layers of dryawall is a good one and will cost less than "acoustical drywall" (80.00/sheet vs. 8.00 for reg) or mass loaded vinyl. I am buildin a basment rehearsal room for myself and that will be much louder than and stereo system. I will construct a room within a room to psyically de-couple the walls and ceiling from the rest of the house. You may ver well do fine with the double drywall and green glue and insulating the walls. Good luck and enjoy your room. There is a good deal of info out there, sometimes too much but this is what I will be doing and I have spent a good deal of time researching it.