Those musical greeting cards have nothing more than a simple switch in them, which closes when the card is opened. You need to replace that switch with a tambourine-controlled switch. Most tambourines have pairs of jangles (zils). If you separate one pair slightly by a spongy spacer (to keep the jangles from touching normally - you also need a non-conducting rod to hold them), and connect a wire to each jangle, you will end up with a switch that only closes when the tambourine is hit. That would work fine for detecting when the tambourine is played, but you have the added complication that the power must remain on long enough for the card to actually play the whole sound. You will probably need to have the tambourine switch hooked up as the trigger to a simple 555-type timer to keep the power on long enough. You should be able to find plenty of 555 timer circuits on the internet.
MIMForum Staff - Way North of Baltimore