After upgrading to Big Sur from Catalina, I remarked my fan was running continuously. Opening Activity Monitor I remarked that "com.apple.SpeechRecognitionCore.speechrecognitiond" was chewing through CPU at 37.3%.
Looking a likely candidate, and just being a bit anal about having a microphone in an always-on state, I wanted to knock that nonsense on the head.
There was a microphone graphic on my screen which presented the option to "Sleep" which when clicked then presented the option to toggle the mic to "Wake Up". Clicking this did nothing: Speech Recognition nonetheless kept the fan humming and chomping through CPU cycles.
After toggling every application's access to the microphone to deny access, "com.apple.SpeechRecognitionCore.speechrecognitiond" was still happily chomping through CPU.
Where is a setting to toggle the microphone off at OS level in Big Sur?!?!?!
Best Answer
This drove me bananas. Should be straightforward, but it isn't.
Googling returned lots of results for disabling the microphone on an application-specific basis in Big Sur, but absolutely oogatz on a global toggle to kill the microphone on a GLOBAL basis to both stop Speech Recognition from chewing up my CPU as well as shut down an open mic.
I next visited "System Preferences" > "Sound" which seemed a logical place to tweak such a toggle. Nope.
It's actually buried in "System Preferences" > "Accessibility". No, I'm not joking; that's where it lives. Untick "Enable Voice Control" and if you go back to Activity Monitor, you'll see you get your CPU back and the microphone should be down.
Another thing that really irks me is that Apple is supposed to telling you when something is accessing the microphone or camera, but you don't get a warning when Apple itself is triggering the microphone. ANYTHING that accesses the microphone or camera SHOULD trigger a warning light IMHO.