I've been using Arduino in conjunction with the sound playback software Qlab, in live theater. Communication has been over MIDI, using the simple hack of the Arduino's serial port as a non-class-compliant MIDI out.
In a current show, the Stage Manager has a "Go" button, and the computer for playback of sound cues is across the booth. Besides getting a big, debounced button that doesn't take up a lot of desk space, it also means Qlab will fire the cues whether it is focused or not.
In a couple of previous shows, I ran a second button through snake or house microphone wires all the way back to orchestra pit so conductor or percussionist could fire off a sound in tempo with the music.
In one show, I used Qlab as a primitive software sampler; an "intercom" button onstage operated by the actor played a buzzer sample from the sound computer as long as it was held down. Then the sound operator in the booth would hit a second button to fire off a pre-recorded voice-over-the-intercom cue (or a backstage microphone, rigged to the same on-stage speaker).
The advantage to going over the sound playback computer, instead of a sampler or similar, is that we can tweak the sound file, and it plays back multi-track, assigned across possible multiple speakers located all around the stage. So instead of being stuck with where a doorbell or live telephone was physically wired, I can play and tweak the sound to whatever works best within the total sound picture -- and tweak it right up to opening night.
I love that I can wire any switch contact or sensor, route it through whatever wires are available in the building, and debounce it and condition it as necessary in software.
And on the Arduino side, a few minutes with a laptop is enough to tweak the code to send MIDI noteOn and noteOff events, or even MSC (MIDI Show Control) Go and Stop commands.
My future plans -- when I get some free time! -- are a dedicated controller surface with big tactile "Go" and "Stop" and "Rewind" buttons -- a cell-phone sized controller pad I can put anywhere where it is reachable but out of way of scripts and so forth.
And also to explore options for spitting MIDI to computer without needing a USB interface; either USB port, or host-side software interpreting a straight serial out.
Since many sound boards and light boards speak MIDI as well, I can use the current Arduino box independently. Since I built my second MintyBoost, the battery lasts for days, too.