Does the film have a 5.1 soundtrack? If you're very lucky the dialog you want will be on the centre channel. Wavelab 5 has multi-channel audio capabilities and is designed to work with DVDs (although I've never used it for this purpose so can't offer any detailed help, or even certain knowledge that it will do what you want). I suggest you start there, and look for an "Import 5.1 audio" function.