Hi Emma,
I was just playing with this in Moodle 3.6, and happened to notice, that when you look at the grading page, the new, Training, theme is the only one that seems to left align the recording without making the Feedback comments column expand like crazy, and center align the recording. Boost and Clean both display the recording center aligned in a very wide Feedback comments column. Do you see the same results, or is it just my 3.6 doing this?