HI, Bclog,
It may be a glitch that is related to the update, in which case we will need to wait until Apple addresses it. Meanwhile, see if deleting preferences helps.
To delete preferences, hold down the Option and Command keys and select to delete preferences in the box that
appears. iMovie will open in a new library. Reopen your old library to get back to your projects. See if the audio works properly.
Another thing you can do is to use your Time Machine backup to restore your previous version of iMovie.
-- Rich
-- Rich