When I first started using iCurveball I had the same concerns, but it was all for nothing. The sound of the phone has never interfered with the handling of the effect or the method at all. Most people keep their sounds off, especially in restaurants and if you're performing for high-end corporate clients they either mute their phones for the engagement or they won't let you borrow it in the first place. If that happens, simply return it to them and try another.
Or, begin the set with one spectator (perform a different effect, like Final Destination or iBalance) and if you don't want to mute their phone, you can give it back and perform iCurveball with someone else. Problem solved :)
But at the end of the day, if you have to, just mute the phone and switch it back on before you hand it back. No harm done ;)
