With capacitive sensing you can probably get some reading of closeness at no more than 1 or 2mm distance (I get some feeble activation with Trill Craft when cranking the gain all the way up). Capacitive sensing can also get confused when the surface gets moist, so you may have some small activation even when not actually touching the surface, so I wouldn't recommend it for proximity sensing in this context. If you want to sense touch it could be just fine in terms of capacitive reading, as you'd get a large differential reading there. However, in the (likely?) case that the microphone case is grounded, and that ground was shared with Trill, you couldn't use it as a capacitive sensing surface.
You could use some ultrasound distance ranger (e.g.: HC-SR04. There is an example in
Sensors/ultrasonic-distance, and someone (maybe you?) also did that in Pd, although the C++ example has more spatial accuracy (because message-rate digitals in Pd are quantised to 1 every 16 samples).