You're right that this room, while realistic and fine enough for an establishing shot, is dreadfully boring for a major emotional story beat.
This sounds like an uplifting story that could be inspirational. I agree it is very challenging to express what is supposed to be a drab setting in a non-boring way. Also expressing the power of song visually makes it a double challenge. So just brainstorming with you here ...Perhaps rather than thinking of it as drab your keyword could be conventional. I can see that you have a chalkboard like it's a classroom or a practice room for the cantors. So all of the furniture is no-nonsense, practical, just like the cantors in suits. So the colors don't need to be boring, but the shapes would all be very solid and square. Or you can tone the colors down and have patterns but they are all very conventional patterns like the rugs in these scenes.
Maybe to set the tone "before the song" there could be a window with a cloudy sky, and then the sun breaks through the clouds while the main character starts singing. This could fill the room with some light and color and also have a spiritual symbolism while "spotlighting" the main character. Or you could go all out and add some colors and abstract flowing shapes in as suggested by @NessIllustration to express the emotion of the song. For some reason this short "Inner Workings" popped into my mind, even though it's not exactly what you are doing. Also I came across this "music room lighting upgrade" image in case it inspires some ideas. Hope this helps a little!