Here's an idea... For mono, do the usual, the circle and bar. If you were using it in a block diagram you'd get by with one pin coming off the circle at the point opposite the bar's contact with the circle, otherwise two, each slightly offset from that position, and parallel.
For stereo, use the two overlapping circles, and a bar joining tangents on each and extending to a point at each end coexistent with a tangent on the points most distant from each other on the double-circle pattern. (Bar is this same length now). Add one pin to each circle as before, for block diagrams, and for schematics, a third extending from the circle intersection point out to a point halfway between the outer ends of the other pins.
Stereo mics do exist, so this symbol makes sense, it fits existing conventions, and isn't going to get confused with a current source symbol because that has no bar, and its pins are in line, not offset.
I could have just drawn this, but I have no file hosting arranged, and figuring that out is a lot harder than saying this stuff..