The kernel that ships with Ubuntu 13.04 is defective. There is something wrong with the 3.8.x series of kernels and it affects people using the open source radeon audio driver. The solution I've found is to upgrade to 3.9.x and add "radeon.audio=1" to the kernel parameters line. Unfortunately the broadcom bcm43xx driver isn't compatible with those kernels and fails to compile. Aaah. Just "radeon.audio=1" on a 3.8.x kernel causes it to panic at bootup.
Also, if you're having video that mysteriously plays at a much higher framerate than usual, it's probably the same problem.
There has also been other weird kernel panics I've yet to investigate. Both at startup and shutdown. Something is absolutely not right with this Linux version.