Crazy things you can plug into your iPhone's audio jack
tuaw.com
tuaw.com
I'd rather pay through the nose and get something proper for the job.
That goes for those POS card readers as well
Apple still have to follow global supply and demand even if they do a lot of demanding. If there was a shortage of an audio interface, DAC or filter IC then it's inevitable that an alternative will be sourced to avoid final product shortages which would impact the bottom line. These may or may not have the same specifications. To a human ear it may not matter but to a coupled circuit, it may be different.
The 3.5mm audio jack is also an incredibly iffy electrical standard. There basically isn't a standard.
Isn't it a microphone jack? Apple can't change that, or microphones won't work.
These programs probably just send frequency tones while pretending to be a microphone. It's not very high bandwidth but it works well enough.
Many of these devices are, essentially, modems. They modulate the analog signal in ways that are tolerant of the faults typical in analog systems. Fortunately, they've got two things going for them:
1) They're sending the analog signal over a very, very short distance.
2) Modulated communications are a very well developed field of signal transmission.
They're hacks, but they're old, established hacks.
They may use the principle of a modem but there is no established standard for communication. I assume that you know about impedance and loading based on the above? If the internal resistance of the current loop suddenly flies up or down, is the circuit on the other end going to perform to the same? Nope. There will be less current available for the device. At that point it will work intermittently or stop working all together.
Your microphone or earphones will still work quite happily though.