Thanks, Nantonos. This is just the sort of feedback I'm looking for. I'm not aiming for the studio musician crowd; I figure they would prefer a self-contained box with built-in power supply, etc. My target market is general Arduino hobbyists and educators. I think if this shield is flexible enough, it could be put to a lot of creative uses, which users could share.
I was thinking more of a PC kind of mike connection. But clearly, I need to do some homework and understand microphone technology a little better. For the ADC and DAC, I was already assuming some kind of external device.
I'm planning to have the smarts reside in an FPGA rather than a microcontroller, mainly because I have more experience programming FPGAs, so hopefully I can come up with some unique applications that would leverage the advantages of FPGAs compared to MCs.