All sound playback from the hard drive on Ubuntu 8.04 (fully up-to-date) will cease to function after a random (as of yet unknown) event. However, sound from sources like Flash (streaming) will continue normally. All playback through programs such as Exaile, Rhythmbox, and Totem will fail, apparently loading the file correctly but simply failing to start playback. My first thought was a conflict between sources attempting to play audio, as I know some people cannot play more than one sound at once, but numerous reboots shows that the error will occur before any sound other than the Ubuntu login music has played. This is a new problem, occurring only in the past week, or so, and having gotten much worse as of late. Again, sometimes (maybe 30% of the time) sound playback will continue normally, with no interruptions, for hours on end, while most times the playback will fail on the first attempt and require another reboot. From there, playback, if it works, will usually only last a few minutes before failing.
Thank you for your bug report. It seems like a bug in pulseaudio. Reassigning.