Today’s update for Ubuntu 9.04 (Jaunty Jackalope Alpha 4) broke the audio mixer and the volume control modules. This update was for in kernel 2.6.28.8. Technically there was nothing modified in the audio modules, but the updates changed some of the user settings which disabled the user from accessing audio modules.
The workaround for this problem is quite simple. In the Terminal run the following command:
sudo adduser your_username audio
Restart the computer and you have access to all the audio devices like before.