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Not sure if this will help but I was having the exact same problem as everyone else. I was using the dictate function for Word on my MacBook Air with no issues until one of the latest updates. Now it turns off within seconds. What I did was I went into System Preferences and instead of going to "keyboard" where I already had the dictation turned on, I discovered there were voice control settings under "accessibility."
Under "accessibility" there is a heading called "motor" and under this there is "Voice control". There is a checkbox called "enable voice control". When I checked this the microphone icon came up and I have been able to dictate with this ever since without it shutting off.
I haven't gone in and out of word or shut my computer off so I'm not sure if this will continue to work; However for now it seems to have overcome the problem..