Dear,
I offer you to make a software (client) that convert the data in frequency tones (modulation), next this tones played to audio output in the client. Next, in the server other application that receives the audio from the mic, and demodulate the audio to data again.
I'm engineer with more than 15 years of experience programming software to different applications.
I can make the software in windows C++ or Java. The last one has the advantaje that it can be executed in all operative systems.
Please any question contact me to can hepl you.
Will be a pleasure work together.
Reagrds
Eng. Juan Manuel Garin