The sign and the ability to monitor hopefully would work.
Do not turn the sound system level above 0 db unless you want everybody to lose the privileged of the sound system entirely. Violations will result in more effective countermeasures an/or the loss of the use of the room. See the contract you signed.
Timers such as these:
https://www.redlion.net/product/cub5-preset-timer-cycle-counter-backlight-display exist and MAY work. This one apparently has an output when the amount of time preset is exceeded. You can also probably get resettable hour meters without the timer for cheap. You can buy stuff and use some "glue logic" to put it together.
The "glue" in this case would be OR-ing the left and right channels and getting an output above 0 db to gate the timer and a Key reset and possible tamper alarm.
If it doesn't work, move onto the next phase where power is removed and manually reset. If a second comparator is available, then say after 15 minutes, power to the sound system stays off and cannot be reset.
Contract example:
If the amount of time exceeds 5 minutes of time above 0 db you will be charged $100. The $100 will be held in escrow until the event is over and the meter read and reset.
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You can't enforce something that you can't measure. You would be measuring the input to the amplifier. That only works if the amp has fixed gain.
Sound or noise levels have an ear perception level associated with them. Defaeat would be removing power to the monitoring gizmo.
In your design, think of how one might defeat it. There are breakers with electronic trips. You could mute the amp with a wireless signal. UPB or Insteon is possible too. You don;t have to really kill power to the amp. You could mute the signal wirelessly or through power line communications.
I'm just "thinking outloud". It may not be an effective design, but rather a place to start. Commercial products with "glue" is generally easier to maintain.