Try switching audio modes. In most of the cases, the problems are caused by the audio codecs installed on your Mac. However, the music you purchase on iTunes might be protected by DRM. Even though you've purchased it, there is no way to make copies of it, therefore you have to de-DRM it.
Search through the database of Mac Informer for applications that can remove DRM, then you will have the possibility of using them in your DVDs: http://macdownload.informer.com/search/remove+drm+itunes