[douglas]

Look in settings.txt near the bottom for a section starting with "Num Intro Songs". Following that are lists of file names of music to play in the intro, during normal play, and during combat. You can add as many as you like, just make sure you keep the numbering correct and update the "Num ___ Songs" field for however many you add.

I'm not sure, but I would guess the music is automatically randomized from the appropriate list in settings.txt.

The sentry order gets the ship out of the next-idle-ship list for a minimum of one turn, but puts it right back in any time a non-friendly presence is spotted in the same system (Note: this is for simultaneous movement mode; in sequential movement, the sentry order might be immediately cleared the instant you give it if a hostile presence is already there). To permanently remove a ship from the list of idle ships until you manually clear its orders, tell it to move to its current location and repeat orders. In sequential movement, you will probably have to move it far enough away that it can't get back until the next turn and then issue the move/repeat order.