Siri is unable to hear me
I encountered that bug again and solved it with the following Terminal command:

sudo kill -9 `ps ax|grep 'coreaudio[a-z]' | awk '{print $1}'`


Within 5 secs Siri works, no need to log out / reboot.

Can somebody please check whether this solution works for you as well or not. And maybe someone can come up with a clue of what to do with audio drivers to stop this from happening in the future?