In addition to applications in audio engineering, and music, there would be lots of applications in electrical engineering, robotics, and physics for a low latency audio interface for Mathematica. That's because the audio inputs can also be connected to all sorts of electronic signal sources, not just microphones. I have pitched this to company developers repeatedly, but obviously not effectively.

PS. Last year there was a discussion in this forum on audio recording and there was a suggestion to try "sox". Here is a demo showing that once you have installed the sox package, from source forge, then you can call its recording function while you are emitting sound. This is not quite as nice as it could be, if built in to Mathematica. The following code fragment works on OSX 10.9.5 and Mathematica 10.0 and is quite reliable, although not 100 %. This example emits a tone burst from the Mac speakers, and records on the built in microphone simultaneously.

f = 1000; (* freq of emitted tone in Hz *)
EmitSound[Play[ Sin[2. Pi f t], {t, 0, .1}, SampleRate -> 44100]];
Run["/Applications/sox-14.4.1/rec -c 1 -b 16 -r 44100 temp.aiff trim 0 .1"];
rez = Import["temp.aiff"];
 DeleteFile["temp.aiff"];
rez