Do you have
anything plugged into the Output jack on the computer? Mine is right next to a similar jack and it is easy to plug into the wrong one.
Anything plugged into the 'audio out' will silence the internal speakers.
When I tapped the (right side of the keyboard) Command key twice, a microphone icon popup box would appear to indicate I could start talking.
Where did you get that command? It may work, I've just never heard of it.
There
is a built-in keyboard command to "Speak selected text..." if you enable it: Option+ Esc
Did you perhaps change that keyboard shortcut, assuming you had that enabled?
1. While you have the Sound Pref open and "Input" selected, do you see any change in the "Input level" when you speak? Don't wake hubby, but the bars farther to the right should darken for a bit the louder the noise the mic 'hears'.
2. I am assuming you have the Volume showing in the Menu Bar. Does it also show some distance from the left side of the scale?
Another System Pref to check: Accessibility->Speech
3. Try selecting a different "Voice", the one you have may have laryngitis.
Might as well make sure all the other pieces of that Pref are still at default values: General, VoiceOver, Audio, Dictation, Speech, Keyboard, Switch Control even Siri
"Default"
generally means
UN-checked check-boxes. check...