I like a lot of Mari Okada's work including Anohana, Maquia and O Maidens in Your Savage Season. However, I came out of "The Anthem of the Heart" with very mixed feelings.
It is a high school drama that feels a bit half-baked. The drama mostly revolves around the main girl, Jun Naruse, who cannot speak because of an event that occurred in the past. This is an interesting premise but the problem is that this is not the main focus as much as it should. Instead, we have a lot of romance subplots with other supporting characters that just aren't developed enough. The third act and even the ending felt incredibly forced as well. I do give props for not being predictable and being a decent message but the execution is kind of poor. This premise feels like it would be better as an anime series than a movie. That way, the supporting characters and even the romance could be more developed.
The movie does have great animation, character design and music. Unfortunately, if you wanted a good high school drama or romance, this is not it.