Very neat! I had a https://www.crowdsupply.com/wee-noise-makers/wee-noise-maker... but I don't always carry it with me. I do have my phone though most of the time in my pocket so having this on, Web based, is great! The author of ToneJS is very kind, if the documentation wasn't clear you might want to reach out to help clarify it.

PS: didn't check it but being a PWA to work offline would be quite neat, just in case the subway or train goes through a spot without connectivity.