Thank you for your reply, and I managed to get the audio works today.
I booted up the board with the kernel I complied, which is based on BSP 11.05 downloaded from freescale and I also linked the drivers for SoC audio (SGTL5000 and S/PDIF) statically to the kernel. The RFS I used is the lucid for 11.05 BSP downloaded from freescale. With this configuration I can play some audio files, with some issues:
1. I play the audio files by just clicking the file in Ubuntu Desktop. Wav audio file playback seems fine, but for mp3 playback the sound quality drops considerably, as if the file is played at a very low sampling rate (I am not a digital audio guy so it is just my guess). Maybe the default lucid RFS doesn't have proper MP3 codec installed, is it correct?
2. On the Ubuntu Desktop if I click System->Preference->Sound, I got a dialog saying "Waiting for sound system to response" and it just stay there. And I also cannot control the volume on the desktop panel. Does it mean that the audio driver is not installed properly so it cannot control the codec chip on the board?
3. Is the HDMI daughter board with sound output? I have the board and according to the schematic the SPDIF interface of QSB is connected to the HDMI transceiver. I am trying to make the HDMI audio works but to no avail. Do I need to do some additional configuration steps to make SPDIF the default audio device?