I designed a pocket music instrument. I partnered with a company in China called Seeedstudio, they do the manufacturing, shipping, customer handling, etc. and I receive royalties on each sale. I varies from month to month, but above the 500$/month on 2025 :)

I love to be able to focus on the design and not the practicalities of selling a hardware product!

https://minichord.com/

I'm a recording electronic musician and clicked through with interest. Maybe I overlooked it, but I didn't see where I could hear how it sounds. It should be so obvious that I can't miss it, so even if it's there, I'd suggest making the link more prominent.