Could've used a speaker and microphone with an appropriate noise-resistant digital encoding.
Gut feeling tells me there is a way, if you use way more power than normal for this :). Much like with making speakers receive sound (you need to amplify the received signal afterwards) and making microphones produce it.
But it doesn't really matter whether or not you can reverse the analog signal flow, if the digital side treats the I/O pins as unidirectional.
The threat model where you use a data diode presumes an adversary might totally compromise the sending side - the guarantee you're trying to add is that whatever malware they push down the line has no ability to exfiltrate data regardless of how compromised it is.
It may be possible to increase the bandwidth by increasing the sample rate on both ends, but this quickly leaves the realm of consumer audio equipment (and consumer pricing). At some point you'd exceed the reasonable frequency responses for each device, as well as the medium. I imagine that air attenuates ultrasonic frequencies more than lower ones, but that's just a guess.