I am doing my reviews in Firefox 94.0.2 on Ubuntu.
Since a few days I have noticed that sometimes there is no automatic audio at some vocab items in the review session. Today for example this happened for 募集中.
When the same item occurs in another review session the audio works. It also works when I click on ‘kyoko’ or ‘kenichi’ in the item page.
I’m also using Firefox, for me, the very first item in a lesson session always has broken audio. Same as for you, it fixes itself when I manually click the play button.