The tones produced by a musical instrument vary with the density of the fluid through which the sound travels. Rebuild every instrument to produce the desired tones in the density of water. Assure that the component parts can withstand mild pressure and contact with water.
Seal the performance
hall so that it can be filled with water, and equip each orchestra and spectator seat with two air supply masks. Also provide air masks for standing players, conductor, and doorman. The air masks include extensive baffles to significantly dampen the sound made by rushing air. The masks also include exhaust connections so that rising bubbles do not obscure the (already limited) view for people in the back seats. The second air mask is auxiliary.
Fill the hall with water filtered for particulates. Spectators and musicians can enter by ground level before filling or by splashing in from above to make a dramatic entrance. Illuminate the stage primarily with blue light so that the absorption isn't as much of a problem.
The music should travel better in water, so the orchestra should produce more visceral feelings. It may be possible to project sound more clearly to the back of the hall.
The musicians would not get so sweaty from heat-radiating lights and the exertion of playing, since the water could be of comfortable temperature. Flash photography would cause less distraction because the water absorbs much of the light.
In case of fire, the fire would cease to exist. There also can't be disturbances from cell phones or pagers, and underwater video cameras should be easier for the doormen to locate, since those devices may be larger to accommodate seals and sturdier construction.
The conductor's clothing would dance even more fluidly and gracefully than usual, giving movements lasting character. Ladies (and gentlemen) with long hair are politely given hair elastics to minimize obstruction of sight.
Additional seating floats above the existing seats in previously unoccupied regions on the ceiling. These can only be accessed when the tank is filled. The 'hanging' seats, suspended by metal beams padded to prevent accidental injury, minimally hold the viewer, employing a seat belt and a long cushion. Any persons who have dozed off during the performance while seated in a hanging seat will find justice meted, dangling in midair as the water is emptied after the performance.