If your gliders are used to the music it should not be a problem for them unless he has an amp turned up very loud, not usually necessary for practice.
My gliders do not seem to object to my playing and singing in the evenings and they do not wake up when I practice during the day. I usually practice in the next room but I am only about 6 feet from the first cage.
The glider may however remain hiding in their pouches if the lights are on in their room so he can read the music. My gliders will not come out if the lights are on in the room. The are not bothered by the light from my laptop or a small TV.
I have actually copied most of my music to PDF format and have it on an Ipad (saves shuffling many sheets of paper) and I can read those quite well in low light.