You could use one of the protocols adapted for packet Ham Radio - there is already a lot of code and they are proven in noisy environments. Whoa I found this page and it would be sweet if we could encode Bitcoin transactions into this:
http://wb8nut.com/resources/olivia-1000-32.wav(from
WB8NUT's Digital Modes Information Page)
These are meant to fit in a very small bandwidth, though, of a few hundred - 1000 Hz. For short transactions it would probably work!!!
You probably wouldn't get more than about 8KHz in real life unless you were directly connected, forcing a max transfer rate of only a few kbits/sec. Directly connected would probably mean modem speeds depending on the ADC. Remember audio in PCs and phones rolls off at 22 kHz.
The problem with the serial cable is that if they do it wrong, their wallet is exposed to everyone. They end up with less security than they thought they did, and the false sense of security be worse than not having had the option to begin with (they would've relied on something slightly less optimally secure, but with less user error).
On the other hand, if they do the audio wrong... no one gets to spend the offline coins. Well, the authorized user will...eventually (even if they have to switch to some other method). You point out that they could blow up hardware... okay that's a valid risk to take into account here.
Not quite correct - remember it is still a digital codec on both sides, so no matter what you will still have the chance to introduce bugs. Even if the hacking now involves some serious sound equipment.