Just in case this helps someone else...I have a new installation of md-12, mostly works fine, except some videos have no sound. To fix it:

1. Open a terminal
2. su to root (don't know if this is needed)
3. Start alsamixer

[root@nick var]# alsamixer

4. Toggle the mute switch on each channel by pressing the "m" twice (i.e. mute then unmute, or, unmute then mute...whichever applies).

5. Now anything I play works...very strange.

Here's the thread I found it in:

http://www.gossamer-threads.com/lists/mythtv/users/441325?search_string=pulse%20audio;#441325