The OP of this closed, unanswered, and unsolved thread:

w-w-w.linuxforums.org/forum/gaming-games-multimedia-entertainment/126344-trying-play-youtube-videos-second-life.h-t-m-l
[darn bot chastised me or posting a url to OTHER sites. Guess the bot ain't too smart]

had precisely the same problem I do. He was running OpenSuse (which never worked at all for me) and I am running Sabayon, the current version of the 32 bit flavor. System Monitor shows
Kernel Linux 2.6.33-Sabayon
Gnome 2.28.2

It's an IBM board with a dual core Pentium D running at 3.4 GHz with 2 GiB or RAM. I've tried the latest versions of the official LL Second Life main linux client and Emerald.

In a nutshell all the sounds work except the audio portion of movies played in SL. Any ideas? Has anyone been able to get the sound to work in movies played in SL with ANY client in ANY type of linux, BSD, or other free OS?