Description of problem:
The sound crackles when using SDL_mixer using programs, see bug 474745. I can reproduce this on my fully up2date F-10 x86_64 system.

The funny thing is if I edit /etc/pulse/daemon.conf and then
uncomment these 2 lines:

realtime-scheduling = yes  
realtime-priority = 5  

And then kill pulseaudio and restart it, the problem goes away. Even though when started from a terminal pulseaudio complains it cannot get RT permission and according to top, indeed it has not gotten realtime permission.

This is not a glitch, re-commenting the lines and restarting pa again, reintroduces the crackling. The crackling happens the most with certain usage patterns. For example hovering the mouse over the X to close a window, so that it gets highlighted, causes the crackle. Note I'm not using compiz, just plain metacity.

It seems that these 2 config options change pa's behavior in some (possitive) way even if it could not get RT priority.

To reproduce do "yum install creack-attack" and then either start crack-attack, or play its music using the SDL_mixer cmdline player:
playmus /usr/share/crack-attack/music/prelude.xm