Unlock a redstone repeater after a period of time

To do exactly what you described, you can use a silent non-retriggerable pulse extender:

The lower pair of hoppers (facing into each other) has 1 item, which decides in which directions items flow in the upper pair. The upper pair (also pointed into each other) has 26 items, which takes 20 seconds to be sent back and forth between the two hoppers. The torch is deactivated when a pulse from the observer comes, allowing the lower hopper to transfer its item, and thus, the timer to start, and will prevent it from restarting if there are no new pulses. Still, every 20 seconds the observer keeps getting triggered, the sound will be made.

What you're looking for, probably, is a simple retriggerable pulse extender.

As long as the observer keeps generating pulses over and over, the signal is active - and the command block activates only when the signal activates first, remains inert until the signal is removed and provided again. That way while the observer 'goes crazy', only one sound is heard. If the observer is off for a second or so, the circuit powers off and will trigger next time the observer does.

Let me add, for interrupting a circuit, like you did with a pair of command blocks, the comparator in subtract mode serves better - insert full strength signal into the side of a comparator and it will stop passing the signal -

...and what you tried to achieve in the beginning before the delay is known as RS Latch circuit, one of simplest memory cells, and it's much easier achieved with two torches or two comparators: