I had the same problem on my PS50B450. No software upgrade was required to fix it.
First press the TV button on the remote,
then enter Menu Setup Plug&Play (only available in TV mode) - at this point the firmware may ask for PIN (0000, 1234, 8888...by default)
Then at the next step select Home Use.
If you leave Store Demo mode turned on the TV will forget most of the changes made any time you turn it off.
In Home Use mode you can turn off the melody following the steps previously posted.