If mikedt's suggestion of restarting/rebooting doesn't clear the problem you need to eliminate your built-in microphone as a next step.
A number of ways of doing this :-
1. If you have earphones which include a microphone plug them in and make a call.
2. Assuming you have the "OK Google" voice option, does that still work.
3. If you have a 'Voice recording' app, see if that still works.
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