Garage Listing room

Without modification it's probably going to be a fairly dead sounding space due to all the exposed insulation. Are you willing to do modifications or would you need to use the space as-is? 

Use 5/8" sheetrock, it blocks like twice the sound of 1/2" for almost no additional cost.

That just isn’t true, sorry.

An STC (sound transmission) rating for a ’standard’ wall with 1/2" drywall on both sides is about a 33 STC rating, an insulated wall with *two layers* of 1/2" drywall *on both sides* is about 44, a wall with 5/8" on one side and *two* 5/8" layers on the opposite is about 55. We still have not achieved ’twice’ the stc rating, and we are well beyond what an existing 1/2" on the existing interior wall, and a new 5/8" layer on the garage would achieve, but it wouldn’t be as effective as two layers of 1/2" on both sides, an increase of only 11 STC points.

Can you achieve twice a standard 2x4 with 1/2" drywall 33 STC rating? Yep, you can get to 50-60, but not with a single layer of 5/8". Would it help? Yep, but not in reducing the transmission by 1/2.

My stereo room is above the garage. On the advice of a specialist I was able to add a floating sub-floor when the house was built.  On top of the standard 3/4" plywood floor, I added a layer of roofers felt covered by a 1/2" plywood layer scewed into the first layer, not into the studs. I also had the wiring for the  equipment outlets direct to a sepatate 20A breaker.

Please do yourself a favor, read my first post again. Specifically,

Usually only people who have actually been through this process know these things. So be careful, because a lot don't, but act like they do.

So for example when I say something like blocks twice the sound, that is because when you build it you will notice about half the sound getting through. If I wanted to sound all snotty and impressive I could spout dB specs all day and all night. This is the difference between trying to inform and trying to impress. Really wish the BSD types would catch on.

In other words you're one of those people you specifically warn about.