FYI - Had an XO-1 with build rawhide-xo 20090528 (and F12? kernel). Ran a giant 'yum upgrade' to bring it up to current system level. When I checked it out, I was able to hear sound from an application that used the fluendo mp3 driver. [Speak also produced sound - but other applications I have did not. Am currently tied up elsewhere - don't have time to look into why some have sound and others don't.]
I do *not* know how come sound started working. I checked, and now I have sound (with those two applications, but not with others) on an XO-1 with build soas devxo-1 (it also has been upgraded to current F11 system level). I'm positive that several days ago sound was not working for me. It might have been the system upgrades I did -- or it might have been my own customizations (which I'm continuously tinkering with). I also checked against the Soas2 builds (including 20090621) - they are up-to-current-F11 level (and have my current customization), but they don't produce sound for me.
Thank you, developers for F11 on XO. mikus