as soon as the battery runs out. it depends on how many times and for how long you open the card.
When you open the card, the front face of the card has a tab attached to it in the crease that is pulled out of its slot in the back of the card completing a circuit and it powers a little chip and...
Hallmark USA has finally done it, a recordable card is released.
soundexpressiongreetings.com
Check out http://www.sayitinasong.com
because you are basically greeting the person by saying happy birthday.