I agree that an audible warning is a HUGE improvement over no warning. There is a LOT of visual information when driving 75MPH down an interstate, and noticing a small blinking yellow light way off to the side is not NEARLY as good as hearing some beeps.
I think the algorithm should be:
1. The SBZA system has detected an object (if just this, then a solid yellow indicator and no audible)
2. The turn signals are enabled in the same direction as the detected object (if this too, then blink the visual indicator, but no audible)
3. The car is moving above a certain speed threshold (20MPH?) (if this too, then beep)
The reason for #3 is for sitting at a red light in the rightmost of a double-left turn lane with the turn signal on. I don't want it beeping at me in that situation. I'm not changing lanes, I'm turning left. The speed threshold solves most of those situations.
For me, the no audible alarm pretty much makes the SBZA useless.
I also think a user-settable setting would be fine - no alert, visual alert, visual+audio alert (or some such)
I also think it'd be fine to limit the number of beeps - perhaps 3 quick beeps is enough, then stop until the next time...