To add to the answer already given:
This would require some experimentation, but you could run an audio modem on each of the computers (one sender, one receiver) then TX/RX the audio from A to B.
An example of such audio modem is here
First test is to take the two computes side by side, line up speaker of sender to mic of receiver, and test such package over a simple air-gap without any TX/RX, the following from the documentation:
Connect the audio cable between the sender and the receiver, and run
the following scripts:
On the sender's side:
~/sender $ export BITRATE=48 # explicitly select high MODEM bit rate (assuming good SNR).
~/sender $ amodem send --calibrate
On thereceiver's side:
~/receiver $ export BITRATE=48 # explicitly select high MODEM bitrate (assuming good SNR).
~/receiver $ amodem recv --calibrate
If BITRATE is not set, the MODEM will use 1 kbps settings (single
frequency with BPSK modulation).
Change the sender computer's output audio level, until all frequencies
are received well:
3000 Hz: good signal
4000 Hz: good signal
5000 Hz: good signal
6000 Hz: good signal
7000 Hz: good signal
8000 Hz: good signal
9000 Hz: good signal
10000 Hz: good signal
Various test modes of this mentioned software exists, to test for SNR, example:
2015-02-06 18:12:48,198 DEBUG Prefix OK
2015-02-06 18:12:48,866 DEBUG 3.0 kHz: SNR = 34.82 dB
2015-02-06 18:12:48,866 DEBUG 4.0 kHz: SNR = 36.39 dB
2015-02-06 18:12:48,867 DEBUG 5.0 kHz: SNR = 37.88 dB
2015-02-06 18:12:48,867 DEBUG 6.0 kHz: SNR = 38.58 dB
2015-02-06 18:12:48,867 DEBUG 7.0 kHz: SNR = 38.86 dB
2015-02-06 18:12:48,867 DEBUG 8.0 kHz: SNR = 38.63 dB
2015-02-06 18:12:48,867 DEBUG 9.0 kHz: SNR = 38.07 dB
2015-02-06 18:12:48,868 DEBUG 10.0 kHz: SNR = 37.22 dB
You can test without TX/RX, then add this in once the you are sucsesfully transferring info over the air-gap.