Terminal is an app on your computer - you can find it in your Applications -> Utilities folder, or you can use Spotlight search to find it. Access Spotlight by clicking the magnifying glass in the upper right corner of your desktop, then type "Terminal". Terminal.app should be the first returned result, hit the return key to run it.
In the black Terminal box that pops up, paste the below sentence exactly, including all quotation marks etc.
sudo kill -9 `ps ax|grep 'coreaudio[a-z]' | awk '{print $1}'
and then hit the return key. A password prompt will appear that just says "Password:" Type the password that you use to log in to your Mac - note that you will not see any asterisks or letters appear as you type. Once you are done typing the password, hit return - it will appear as though nothing has happened, but Siri should be working again.
As far as what you're actually doing here - CoreAudio is a background process that runs on your Mac that manages how apps play and record sound. It's very important, so macOS monitors this process to always make sure it is running. I'm not sure why, but the Siri audio problem has to do with some sort of issue between Siri and CoreAudio. The command that you run in Terminal kills the CoreAudio process - but because it is an important process, macOS immediately starts it again. This restarting process seems to fix the issue with Siri audio.
Hope this helps.
