I’ve hit this twice now when the Google App has updated so I figured I should note it for future reference.
Symptom 1, trying to record using the stock voice recorder on my LG phone I get a message that says Cannot start voice recording while other application is using audio. Symptom 2, the alarm clock/timer doesn’t make any sound.
Solution, go to Google Settings -> Search & Now -> Voice -> “Ok Google” detection and turn off the From any screen option.