How about adding an audio CODEC and break out board to SFEs lineup? Something like the MAX9851 or TI’s PCM3793A would be fun.
TI’s TLV320AIC23B is the best 
Low power, in/out 16 bit stereo, integrated mic in, wide frequency adjustable.
TI offer a development board with USB connectivity and windows “USB audio” drivers. It could be feasible to make a board that could be used as a PC soundcard, making debugging, scoping and testing a lot easier to start.