I understand a freemium model, but even with a paid membership, the app requires you to purchase songs you may already own or have access to via Apple Music or Spotify in order to play them. Im sure the app developer earns a commission for forcing you to purchase a song through iTunes, and maybe that helps to cover the cost of creating the vocal guide for it. However, I would expect that should covered by the paid membership. Its rather nasty to put up a paywall to access content, and then, after youve paid, charge additionally for every piece of content.
Also, even after paying for membership content is still restricted until you write a review. There is no way to say "no" to writing a review. You must say "later", and even then, it doesnt seem that content is then visible -- either extremely coercive or a bug. Hence this review. If an app is going to force me to write a review to view content Ive already paid for via membership, then, regardless of the quality of the core features, they dont deserve a great review.
To the app devs: Youd be better off not pushing so hard for a review w/in the app. The incremental reviews you get from pushy requests to review vs a softer approach are less likely to be positive, if only because youve only managed to force some users to review who wouldnt otherwise via reducing the quality of their experience with the app until they do so. Not smart or respectful to your users -- especially if theyve already paid you for "membership".
That Awesome Guy about Singing Lessons - Vocal Coach, v3.1.0