I have used audimute sheets
I have used audimute sheets for years, from a private teaching studio to a college-affiliated community music program. Excellent product at an incredibly reasonable price. All hardware is included, and they are super easy to install. I use the basic sheets and while the sound isn't eliminated, it is dampened to a level where we can have simultaneous lessons in neighboring rooms with no distraction, and the sound inside the room is still bright enough to judge the performance.