As far as I know, the only packages that are different between 2008.11rc2 and 2008.11 are the entire package and one other package which, like entire, is a "consolidation" package and does not contain any files but merely specifies dependencies.
The first thing I would do is keep your old boot environment, and indeed boot into it. Verify that sound works there, as I think it should given what you said you did. Then update your system again, as per the instructions here: http://opensolaris.org/os/project/indiana/resources/relnotes/200805/image-update/ This would create another new boot environment. Now, reboot into that environment and see if sound works. Then, if you want, you can install SUNWgnome-media-mp3 and see if that breaks anything. Although, personally, I don't know what SUNWgnome-media-mp3 is for, since to get gstreamer working with mp3 you apparently need to follow the instructions described in this thread: http://www.opensolaris.org/jive/thread.jspa?threadID=85323&tstart=0 Anyway, that's how I got Rhythmbox and Songbird to play mp3 files on my computer. I'm probably just as new to (Open)Solaris as you are, so I am no expert. But this seems to be a perfect opportunity to take advantage of the ZFS snapshot feature, which is what gives you multiple BEs. I have SUNWgnome-media-mp3 installed btw and did the same update you did (without even creating a new BE, since the update was trivial) and did not experience any problems. -- This message posted from opensolaris.org