I am interested to use Android's Audio socket for modem use. This kind of work has done before. Sparkfun also sells a board. But the android side application/source matter is not clear, or I failed to get it.
Now, is it possible to make the android application (serial - both send and receive) with B4A? Other end can be microcontroller/arduino or pc. Has anyone done this with B4A already? Any help regarding this is much appreciated.
I am also interested in doing a similar project you describe. What I try to do is send and receive audio from a travez dispositovo android device to another android.
So far so good, the problem I've found is that I decodificr received audio-frequency to determine that I am getting, and also know if this is the bit 1 or bit 0, and can not find how to do it from B4A.
Maybe I do not help with this, but sharing information maybe we can clarify something or someone can help.