Can you modulate sound waves? Like can you have a sound wave of a relatively low frequency and modulate it with a sound wave of a much higher frequency which people cannot hear and send it through the air to then be demodulated at its destination?
Can you modulate sound waves?
Of course. In fact, back in my day, if we wanted to connect to a remote computer, we had to modulate our sound waves and then send them through the telephone.
can you have a sound wave of a relatively low frequency and modulate it with a sound wave of a much higher frequency...?
No. It only works the other way 'round. The "carrier" wave must have a substantially higher center frequency than any frequency component in the "signal" that you want to modulate it with.
... which people cannot hear?
You can modulate an ultrasonic carrier wave with a relatively low-frequency signal. I am not aware of any applications that transmit signals through air using a modulated ultrasonic waves, but there have been practical systems for transmitting signals through water since the mid 20th century.
What you can do for electromagnetic waves, eg amplitude modulation, frequency modulation, etc, you can do for sound waves. The problem with sound waves for communication is that they tend to be attenuated quite rapidly.
Here is an example of an AM signal with 500 Hz carrier, 50 Hz modulator, raising the depth of modulation from 0 to 100%.
The $500\,\rm Hz$ carrier was chosen so that you could hear it!
Yes, you can, and there are even things using it (both historical and modern) for communications. However, you need a higher frequency carrier, and a lower frequency modulation (just like with EM transmissions).
The classic example of this in practice would be the acoustically coupled modems that were the norm for early long-range computer networking (and eventually evolved into the directly coupled modems that most younger generations think of when they hear the term ‘modem’). The same principle was also used by ‘modern’ fax machines, though they typically directly coupled the tone generator output to the telephone line.
In more modern usage, some IoT devices utilize acoustics to establish a Wi-Fi Direct or Bluetooth connection with a smart-phone for initial setup. Such usage typically involves a low ultrasonic frequency carrier wave modulated using some digital modulation mechanism (I believe QAM and PSK are the most common cases) to transmit a few kilobits of data (just enough for pairing) over a very short range (usually no more than 3m).
Using acoustics in this way has a number of issues though when compared to optical, electrical or RF communications. I wrote an answer on Network Engineering Stack Exchange a few years back about these limitations that may be of interest, but the short version is that using sound this way puts a hard limit on latency and a nontrivial limit on effective range.
There is quite obvious example and it is called "speech".
The modulation scheme is rather complex and multiple standards are in use as of today, but the method is at least 500 million years old (including simpler schemes used by other species) and some legacy has accumulated in the meantime.
Not only can it be done, there are commercial products using the idea
The idea of modulating a high frequency ultrasonic sound wave with audio-frequency signals was once thought to be an interesting way to create new types of HiFi speaker. This idea failed because it turns out that ultrasonic signals have very narrow beams and this doesn't really work in domestic HiFi where the audio signal might only work if you sit in exactly the right position and don't move.
The idea has been explored since the 1980s for various purposes. There are several ways to demodulate the signals including interference between multiple beams. Some non-linearities in how strong ultrasound interacts with air and surfaces can also work.
But sending audio signals to a specific place without annoying other people nearby is useful in certain contexts. And this is what the commercial applications of the technology exploit. This is what the product Audio Spotlight achieves.
So, high frequency sound can be modulated with audible sound frequencies (not the other way round) and there are real examples of the idea in use now.