Re: How does the 'musical' birthday candle work?

Not sure I can't answer you with certainty.  We have had one of the 
candles that you talk about.  The one we had contained the battery and 
music chip inside the base of the candle.  As you say, that part is 
similar to the ones found in cards.  Now all you need is a way to close 
the circuit.  Two ways I can think of is to use a small bi-metallic snap 
switch which will close when the heat from the candle travels down the 
metal strip contained within the candle.  The switch is similar to the 
ones used in thermostats or safety switches in your clothes dryer.  Since 
the chip is a fairly complex circuit, it would not be hard to add a 
temperature sensitive circuit that turns it on above a certain temperature.

Short of taking apart the unit under a magnifying glass or Xray, I know of 
no way to be sure other than the write to the manufacturer.