There isn't currently any resolution for this problem other than to exchange the nook for another. I advise testing the new nook at the store, it took me two tries to get one that didn't have the problem. The quickest way is to use B&N's wifi to stream Pandora, make sure you have sound and then stop pandora and set it down for 2 minutes till it sleeps then unlock it. If you don't hear a "scruntch' noise when you swipe to unlock it's one of the broken ones.
I've read, not sure how reliable they are of course, of some people having to exchange 3 or 4 times.
Some stores, reportedly, aren't taking exchanges on Nooks with the sound problem. Mine did without any issue but I'd only had it a week and still had the receipt. YMMV.
It's either a ROM or hardware/build issue as the problem persists under Android 2.1 and Android Froyo 2.2 (and of course under B&N 1.0.1) which rules out it being an OS issue.
My original NOOKcolor had the same sound issue. I returned it to the B&N store I purchased it from this morning and got a replacement unit. The replacement unit has neither the sound issue nor the screen flickering issue I saw on my original. Yea!! Both were from serial number lots 200501015xxxxxxx.
A word of warning -- the person who helped me told me it was good that I'd exchanged it this week, because as of January 31st, all post-14-day warranty claims would be handled by phone support (not bad -- just inconvenient). Apparently, January 31st is the end of the "purchased around Christmas" return period. So, if you are having this issue, exchange your unit before the end of this month. This was in Texas, but I assume all the stores would have the same return/exchange policy. My NOOKcolor was about 4 weeks old.
The person I spoke with did talk to their internal B&N support group, and after determining that I'd already updated the firmware and done a factory reset, they exchanged the NOOKcolor easily. I give them an A+ for making it very easy to exchange a defective unit, as it should be.