I wrote: "Can you devise a program to turn the 4kHz tone on and off at a rate of 500-2500 Hz?"
I was suggesting a way to modulate 4kHz in order to generate a lower audio frequency (because you first stated 4 kHz was as low as you can go).
Since you now can play 2 kHz, that's excellent.
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As for what sort of tones are suitable...
A key that is accepted might cause a cheerful sound, example, a pitch that swoops upward in frequency, or warbles.
A key that is incorrect might cause a pitch that drops in frequency as though disappointed.
TV game shows often play similar sound effects.