Seems I'm having the same trouble. I haven't done any real systematic tests; going from memory here...
Sound works fine when booted from XBMCbuntu live CD (Frodo Beta 1 repack). I'm just using HDMI cable between PC and TV. I tried XBMC a few times like this and all was good so I decided to install to the HDD.
I formatted the drive when installing XBMCbuntu. Sound worked fine at first even through a couple reboots. The first time sound stopped working, I had to go into settings and change audio from HDMI to "analog". This worked but seemed odd to me because I'm using HDMI cable.
Sound stopped working again at some point after switching to analog. I think things got weird when the system went to sleep or hibernate (maybe?).
I was getting the "error - no devices found". I flipped over to a login prompt (with xbmc still running) and ran aplay -l. It listed my devices as expected. When I flipped back over to xmbc and went into the audio settings, it displayed some devices but the list was definitely different than it had been before (can't say exactly what was different as I was just dickin' around with stuff trying to get it to work - device list was shorter I think). I was able to select an NVidia device (still set to analog) and sound worked again - this was last night.
I left the HTPC on last night but had to switch it on using the power button this morning (hibernate?). Today, no sound no matter what. I've shut down and/or rebooted numerous times - always get "error - no devices found".
I haven't tried anything in this thread yet. Sorry for the ramble; hoping my observation might shed some light.
Here's output of aplay -l. I'm using the NVidia card.
Code:
**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: VT1708S Analog [VT1708S Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 1: VT1708S Digital [VT1708S Digital]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 2: VT1708S HP [VT1708S HP]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 3: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0