It works! After replacing the relays with ones that were more robust, everything is working every time.

I wasn’t expecting the new relays to be so loud–too loud for some applications.
But in a musical performance I wouldn’t think they’ll be very audible.
The box came out pretty sharp, and the box itself was only $2.00 at the thrift store.

I taped a breadboard and the arduino against the walls:

And here’s it running from max/msp (you can hear how loud the new components are):