First, this goes under "highly technical."
Second, you need a device called a "modem" which stands for "modulator-demodulator." This turns data into sound so that it can be sent over an audio link. Even with high-quality equipment, 300 baud would be the best that I would expect for a kludge like this... Note that since this is a single-duplex link, you cannot use a standard computer modem.
They make radio RS-232 links. Google is your friend. Something like this will work FAR better than anything that you could throw together. You might even be able to hit 9600 baud with some models.