While the stereo microphone in the C920 can be good enough for most, for those looking for an improved sound clarity or those who want to play instruments or music in the background without it distorting or sounding like it’s behind a thick veil, a cheap and cost effective solution is the Blue Yeti microphone. Many podcasters in streaming (ustream, twitch, etc.) have used it and, while you won’t be getting studio-grade quality from them, it is still a huge jump in quality from the built in microphone in your webcam.
Currently the Blue Yeti is priced at $109 on Amazon at the time of this writing. There is also a higher spec’d version called the Blue Yeti Pro for around $170.