Cool! But from a musician point of view, it'd be better if the sound would only kick in at the next pattern entry. At the moment it just seems to just turn off mute. Then you could add live loading of samples, panning, fades in etc. :-)
Ha, yes, I'd love to do some of that! Actually, having the sound come in on the next loop should be fairly easy - you can just give a start time in the web audio api for when the sample should play.
If you're interested, feel free to leave an issue (or a pull request!) and I'll take a look!