I would guess that the units used in smoke alarms and microwaves generally have integrated drivers that only operate at a single frequency.

Yeah, you could more easily create a rhythm than a full melody. If you get a few devices, which beep at different frequencies each, you could do a lot more by having them beep in succession and in intervals.

Of course, this requires that they're roughly in tune, which may not be the case at all. 🥴

You can if the buzzer signal is only used to trigger a secondary circuit that does what op is looking for.

Depending on how briefly they can be triggered i wonder if it could be fired in a controlled enough temporal pattern to create recognizable notes. Human hearing goes down to 40 cycles per second, so if it can fire in burst of less than a 40th of a second then that could work