- urn Multiprocess AV off (simple fix), or
- Start using Soundflower to route audio from MegaSeg into Nicecast. (Advanced fix)
The latter gives you the additional benefit of being able to use MegaSeg's Preview and cue channel without it going over the air, as well as the Mic play-thru feature.
More details on using SoundFlower with MegaSeg can be found here: http://www.megaseg.com/faq.html#stream