The problem with being dead and buried - besides all that rotting flesh stuff - is that it's darned lonely to spend all of eternity by oneself.
Thankfully, inventor Jeff Dannenberg took care of the problem with this nifty and patented invention:
An apparatus and method for generating post-burial audio communications from surviving friends and loved ones in a casket by providing a burial
casket, and providing an electronic audio communication system for placement in said casket to automatically electronically generate post-burial communications in said casket.
This way, you can continue wish the dead "Happy Birthday," "Merry Christmas," "Happy Anniversary" until the end of time. Or until the battery runs out, whichever is first.
Link - Thanks Martin g!