Overwriting did not work for me, either. TC's support pages says it can't do anything with ROM files. Not sure if there has been a tweak to the base ROM in this area?
At any rate, I found a work around in this thread:
http://forum.ppcgeeks.com/showthread.php?t=10940
I just disabled the "EnableSound" reg entry by changing it from 1 to 0. It's in the [HKEY_CURRENT_USER\ControlPanel\PhoneExtendFunction \CDMA\FlightModeOnOFF] directory. No more startup sound.
Thanks for the responses.