Just had the same problem, it started playing a different one each time! We had lost the insrtructions but dad worked it out.
1/ Remove the bell push front from its back plate which will be stuck to your door frame
2/ Remove the the silver circular battery for a few seconds then replace it, this will reset the bell push
3/ Remove the batteries from the speaker unit for a few seconds then replace. You have now reset the wireless connection between the bell push and the speaker.
4/ Select the tune you want using the button on the back of the bell push circuit board.
5/ Replace the bell push front onto its back plate
Voila!
Phil
October 2009