getting anything off of an original xbox disc isn't as easy as having a dvd-rom. only an xbox dvd drive can read original xbox games. if you want to get anything at all off of an original xbox game, you need a modded xbox and you need to ftp into it; i'm assuming that yours isn't modded.
the format for xbox audio files will vary from game to game, and most of the time you can't simply extract them (they're usually embedded within other files or in a proprietary format), although occasionally they're in a format like .wav. doing what stu_e said is the only "surefire" way to get a recording of the audio from xbox games.