If the audio was too loud or too soft, you'll want to note that in a review. There may be a correctable problem that will improve the show's quality later. Most acting at a local level is variable at best, so be constructive in your criticism at all times. If a particular performer showed a tendency to scream their lines, you might suggest that 'performer X' work on modulation and interpretation, not get out of show business forever. If some element struck you as exceptionally good, include it in your review.
Good reviews try to balance the good and the bad, so avoid going completely negative or positive.
The editor wants to see clean, readable copy at a decent length. Most reviews are not long, unless they are used as a feature story in the entertainment section. Those stories are usually assigned to staffers, so your review should read more like a shorter book report. Ask the editor ahead of time about the length requirements, so there will be fewer misunderstandings when you turn in your final copy. These are YOUR impressions of the production, so you can be a little less objective than a standard news story.