I can't hear the piano sound of my stude

[Gusiest]

I have 15 piano students, and in the case of students who use iPhone phones, I have the problem that no matter how hard we try to solve the problem using the application tools, we do not achieve anything, as if it were really a configuration problem of the iphones

If I can't solve the problem, I'll have to move these students to other digital platforms, and it's something I don't want to do. I need help regarding this problem