We've seen several microphone issues with 8x8 that are being caused by various issues. However, one that seems to affect all outgoing mic audio is the microphone array settings. This setting keeps getting muted somehow, likely through updates and is causing the outgoing audio sound to become muted.
Solution:
Find your speaker icon> Select sound> Recordings tab> Right-click your microphone and select properties> Levels tab> Check that the sound icon under microphone array has not been muted.
If it is muted click the audio icon to unmute it and then press "OK".