Step 1: Everything you need.
Musical greeting card.
Micro switch (radioshack $1.69)
18 or 20 gauge solid core wire
Spool of Solder
Solder station with helping hands.
Step 2: Disassembly of card
Remove the circuit with battery and speaker.
Locate metal tab that was used by the card as a switch. Bend tab back and forth till it comes off.
Step 3: Configure the switch
Strip about a quarter of an inch of the wire insulation off each end of the wires.
The pressure switch has three terminals; n/o, n/c and C they stand for Normally Open, Normally Closed and Common.
One wire will always be attached to the Common terminal. The other wire determines how the switch will act. Since I want the music to trigger when the tin is opened I'm going to attach the wire to the n/o (normally open) terminal. That way the circuit is open until the switch is pressed.
Slip the wire through the terminals and make an L shaped bend to hold it in place.
You can test that you have the switch configured correctly by touching the two contacts on the circuit board with each of the wires. The music should start and if you depress the switch it should stop.
If everything checks out remove the battery from the circuit to make it ready for soldering.