I mean the orchestral stuff which usually plays at the beginning of the game. I've tried nearly everything and I just can't stop it.
It's also quite distracting when I need to hear stuff in the game.

Help?

don't remember off the top of my head, but things you can do:

1) lower the music volume (2 different volumes, music controls one of them)

2) stopsound (it is a console command, it stops all sounds currently playing. bind a key to it and tap it when you want the sound to stop)

3) Replace the audio files.

Stopsound doesn't work, and I'm pretty sure there's no audio control for it.
There should be a fix for this in future versions.

If I wanted to replace the audio files, what and where are the files?

The ninlegends customization pack gets rid of this, it also has loads of other stuff that would help you out,but if you only want the music you can choose to only fix the music.

If you mean the music soundtrack, type cl_musicenabled 0 in your console.