Converting from PAL to 23.97 NTSC is a complete minefield of audio / video synch issues. If I'm honest, this is a technical point that's beyond my knowledge, but essentially you are looking for a way to create a "film effect".
Dvinfo.net is a forum geared towards the higher end, with some very knowledgable people. They have an entire forum dedicated to "the film look", and I would suggest you take a look there:
http://www.dvinfo.net/conf/forumdisplay.php?f=34