If you can get the actual state of the "Fasten Seatbelts" sign, you could trigger off that instead, but I doubt many addons actually simulate it.
If you're executing a single command twice, you can either:
Have the first execution set a variable to log the last time the command was executed, so that the second execution can be ignored if it occurs within a certain amount of time from the first.
Or, have the first execution wait for a set amount of time, and if there is a second execution, the first is ignored, and the second runs instead.
The delay for either would depend on how quickly you anticipate rotating the knob. If you can do so with a relatively quick flick of the wrist, a 1000ms delay could strike a good balance between false positives and false negatives(Where the command doesn't run twice while it should, or runs twice while it shouldn't).
The main issue with such an approach is that without knowing which way the knob is rotated, the sound would still play potentially when it shouldn't(when returning the knob from "On" to "Off", or "On" to "Auto").
If you can, you'd pass the current position to VoiceAttack, which combined with the delay would determine the behavior.