This is kinda cute. Find an older electronic item (toy, gadget, etc.) that makes a sound you like and think you could make even better. For ease try to find something from the late 80’s or early 90’s that is not too complicated. Also make sure that it can be battery powered when you are trying to modify it. Most importantly, make sure you are not too attached to this item as you may ruin it completely. Link.
How to Modify Electronics