Hackaday wrote about a nifty hack by Joe Grand:

[Joe Grand’s] Toothbrush Plays Music That Doesn’t Suck

It’s not too exciting that [Joe Grand] has a toothbrush that plays music inside your head. That’s actually a trick that the manufacturer pulled off. It’s that [Joe] gave his toothbrush an SD card slot for music that doesn’t suck. The victim donor hardware for this project is a toothbrush meant for kids called Tooth Tunes.…

Joe published full documentation for the project on his website:

• Schematic
• Bill-of-Materials
• Assembly Drawing
• Source Code

The PCB is shared on OSH Park:

Tooth Tunes Hack

Joe describes the project in this video:

Hear the toothbrush in action: