Aside from the obvious consideration of volume, keep in mind the emotional impact that music can have. Because the spect should drive the story, it shouldn’t be used to lead the spect (e.g. manipulating their emotions through tear-jerking chords or a quick tempo), but it is interesting to use it as a backdrop that follows where the spect wants to go emotionally.
The biggest lesson we learned in working with live music – less is more. Silences are powerful when a spect-actor is accustomed to a layer of music coming in and out. Because the spect’s senses are on overdrive for the opening scene, music only complicates the work. Best to let them get comfortable with their surroundings before putting some of their focus on a new element. Tense moments are heightened when there’s a break in the music, when nothing else is audible except the dialogue and sounds of the scene partners, and then a quick start gets the pace moving in a fresh direction.