What is it? Good question. Put the words together and you got it. Add tracks to books. Which seems weird and distracting until you start thinking of the uses for it.... Yes you can add background noises, sounds and sound effects to works of literature and then read along with your added noises.
WAIT! Before you don't like the idea... You can also add your own text. So, think of it this way.
Students can create your own writing assignment/essay/paper. The assignments might be on mood or tone. Now students can copy and paste their work into this app and add some effects to their assignment and see if the tone or mood really matches the added effects. Sounds and effects can get added to different sentences and you can change the speed. So in this sense it could work well. Students can still create the work but use this site to "jazz it up" a bit.
Teachers create an account and then a classroom.
Students create an account and Join the classroom. Content can be shared in the classroom (or publicly) so that students may see other students work and evaluate it.
Drawback - Students work will become public on this website. BUT they can delete it after the teacher grades it or it is presented...
There are more lesson plans and ideas for teachers... Here is sample one for "Invoking Fear" in writing.