To make the low-frequency signal audible, you can also use an up-converting mixer to shift the spectrum into audible region.
The simplest way to do so can also be- use a 555 timer running as astable multivibrator and inject your low-frequency signal into pin 7 via a 0.1 uF capacitor. Adjust DC voltage on this pin with a potentiometer to hear the sound.
You can also use another pulse generator, there are many voltage-to frequency IC converters that can work a similar way.