You'll have to rip the audio file from the DVD (.ac3 or .wav depending on the source), then convert the file to .wav if necessary, then cut out the bits you want, then re-encode to your desired format if it isn't .wav.
My recommendation: DVD Decrypter for ripping Womble MPEG Video Wizard for converting .ac3 to .wav (open the file, export it, changing the audio settings to .wav format) SoundForge or Vegas for editing and saving into whatever format.