I've had a similar problem where it would sometimes think my lineout speakers also have a microphone. This trick doesn't work because it completely removes the "Internal microphone" option, replacing it with a nonfunctional "External microphone" option.

replied here: https://news.ycombinator.com/item?id=9806321 - switching audio to the hdmi device had no effect

Also buried in System Preferences -> Sound -> Output (or Input), along with some additional controls like balance and a preview of the input level.

that's useful.