[Ted White]

If you have no need to isolate the sound then definately don't bother

 

[Ted White]

You're quite welcome

 

[Ted White]

Please don't forget to account for ventilation isolation, the doors and any penetrations like outlets.

 

[Ted White]

For outlets, does a spray foam (such as Great Stuff) work at sealing them? Or is that not enough?

Not enough. Simplest is to slap a putty pad on the back, seal up the front with a bead of sealant. If you have a single gang box, that's sufficient. Double gang boxes and larger require a backer box.

Another question: if I had the budget to do only Whisper clips or Green Glue, which would havethe better bang for the buck in terms of isolation? (Both assuming dual layers of 5/8" drywall)

Not possible to answer. They do different things. Decoupling (clips) don't damp (Green Glue). Different structures would give different results. Some rooms would give better results with GG only. Others would give better results with the decoupling clips.

 

[Ted White]

The issue with these foundation walls isn't the concern that sound would pass through the block. The issue is that sound passing through a single layer of drywall will have an opportunity to enter the joists overgead. Very sneaky flanking path to the upstairs. So we treat the walls even though there is a foundation there.

We want as large a cavity as we can in these instances. While you could attach the clips as described, you would be far better off forgetting clips and framing a wall. 24" OC is fine. Possibly cheaper and creating a bigger air cavity.

More information on this found here: http://www.soundproofingcompany.com/library/articles/triple_leaf_effect/