Although I can see the impetus for your request, I don't understand why we're satisfied with non-descript beeps when even a birthday card can play a song now.

I was once in a high-performance single engine airplane, taxied to the hold-short line, waiting to take off when I heard a persistent beep-beep-beep... it wouldn't stop. I though of the systems that produce 'beeps' on that plane--if you get to landing speed with the gear still up; if the ground prox radar thinks you're too low with gear up, if the autopilot is engaged but set wrong, if the stall-warning flap indicates impending stall, the emegency locater transmitter, etc. All really scary, but I'm still on the ground! I was starting to sweat.

It turned out to be my alarm wristwatch, set for noon, lunchtime.

The reason appliance beepers are high frequency is because they use very small piezoelectric speakers to save space and money. Piezoelectric speakers only work well at high frequencies and small speakers are necessarily high frequency. Also, higher frequencies are louder for their wattage. Low wattage eliminates the need for an amplifier and increases battery life, if it uses a battery. A piezoelectric speaker can produce somewhat lower tones, but the volume would be much lower. But if you can hear low tones better than high tones, you still might hear it better. They could just spend a little more and use voice-coil speakers.

I wonder if a high frequency speaker can be modulated to produce a lower frequency. If it could stimulate a larger membrane that would be sympathetic to a lower frequency it might lower the audible frequency.

Losing the ability to hear high pitched frequencies is natures way of helping us tune our wives out.