Not sure if anyone came across this, or if it had been posted in the past, but the NRC did a TON of tests back in 1995 on various interior wall construction methods and subsequent sound transmission.

Really quite impressive results and really helps to narrow down what makes a difference and what doesn't.

Sound Transmission Through Gypsum Board Walls:
Sound Transmission Results
By J.D. Quirt, A.C.C. Warnock, J.A. Birta
Internal Report IRC-IR-693
October 1995

http://archive.nrc-cnrc.gc.ca/obj/irc/doc/pubs/ir/ir693/ir693.pdf

"Those who preach the myths of audio are ignorant of truth."