Thanks for the quick reply. I believe it's the background sounds that's not being muted...at least I think it might be coming from Chatterer. I am running v1.2.2 1622 build on Win8.1. Let me know if there's any info from me that might help.
And, btw, I might not have even noticed if I hadn't installed a mod called MusicPauser that mutes KSP music when the game is paused. I found that mod at http://google.com/musicpauser-mod/ and was pleasantly surprised that it ran fine with 64 bit KSP.