I'm wondering if it would be somehow possibly to have easy communication with a computer for Arduinos that don't have USB like the pro mini. That would make an easy way of having two-way communication with a computer. The computer could just play or record audio using software written specifically for this purpose. Would something like this require any special circuitry or would it just be a matter of connecting the right wires?
I didn't read the all page, but it seems it can be achieved, see David Awehr's blog (this one is conected to a android phone instead to a PC tho) and this Hackaday page where they seems to it, but the PC's audio input normally use 44,1kHz or 96kHz, so if you wanna go for the higher Serial speed (let's say just 115200bauds) you should think about it this.