ThumbMusic is a digital kalimba, the application requires at least a Palm III with OS 3.0. It is a really small app running in about 3k of memory. If you click on the title of the post you’ll go to a web page about the app and there you can download the source code for it too.
I use this on my clie T425. Which is an old OS4 device, in fact a monochrome one at that, but fine for running older palm applications that won’t run in OS5.