For the music, take your cue from some of the good horror films - establish a "scary" track, that plays when danger is approaching. You'll signal a few scares, but it's setup. After a few repetitions to train scary music = danger and other music = safety, start fucking with the player's expectations - play the scare music when they're safe, have them attacked when the safety music is playing.
Have the scary music play when they're just walking along, keep playing it, build it up and up, then nothing. Cut to silence. Nothing attacks. They walk further and the safety music starts. So hopefully they're on edge, but heartrate is returning to normal, then spring the monster, preferably from behind and completely without warning. Make it loud, make it get up in their face, make them need a fresh pair of pants after the encounter.
Music is potentially very powerful at yanking around our emotional state, if all you do with it is confirm what they already know from what they see (or warn them of what they're about to see) then you're not going to scare anyone. The scare comes when the music says one thing and the rest says something else. And as said, it can also be used to induce panic if you've already got them on edge.
The important part, don't let them feel safe too often, and then every so often make them feel safe right before you spring something, then they'll never feel safe again.