The vast majority of web cameras sold these days already feature a built-in microphone, so finding a Mac mini webcam with one isn't hard. But a few don't such as the Microsoft Xbox Cam, The Cubie or iMage from eCamm, and a few others. Some of the higher end HD cameras for Mac include dual or stereo microphones on each side of the webcam's lens.
Im my testing, most no-name, cheap import webcams are just that; Cheaply constructed with less than optical quality webcam lenses and sensors, and equally sub-par microphones as well. Some of these cheap $10-20 cams had ok or so-so images and microphones that often sounded either muffled, harsh sounding, or hollow and thin. Some use a separate 1/8" analog cable for the microphone which often doesn't work at all on Mac's because of differences in Mic vs Line-Level input voltages. Any Apple compatible webcam with mic should use just a _single_ USB cable for both video and audio. All in all, these cheap import webcams are proof that you sorta get what you pay for when you seek out a dirt-cheap camera with built-in mic from the bargain bin. If your Mac mini webcam needs aren't demanding that might be 'good enough.'
On better quality HD webcams from companies like Logitech, Creative Labs, HP and Microsoft -- We're seeing dual noise canceling or stereo microphones built in to cameras for quality audio capture to match the razor-sharp High-Definition images these premium webcams deliver. Similar to what Apple is doing with directional 'Beam-Forming' mics in the recent Retina MacBooks, dual microphones can use differential voltage signals between the two mics to help with background noise and echo cancellation and give a clearer, cleaner video clip audio capture, or improve an Apple Messages, iChat, Skype or voice conferencing experience.
A webcam with stereo microphones does add ambience and directionality - especially in multi-person chat. But because web cameras are small and the mics are placed rather close to each other on each side of the camera lens, the stereo separation is very subtle, not dramatic.