so after first boot you heard the sound after logging into your ubuntu system, and since that one time you havent heard any other sounds? there must be something you edited or changed after that to result in this, right?
maybe try upgrading 'alsa' (which is like a universal sound card driver for linux).
to do this:
look for a program called 'synaptic' (which is a 'package manager'.. it could be compared to M$' 'add/remove programs'), it is under the menu at the top i think 3rd from the left (if your using gnome, most likely), i forget the name of the menu.. but from there you select administration -> synaptic. youll be prompted for a password once ran.
once in the program i think theres a 'reload' button which will look online for new packages or upgrades that are now available, click that first.
there should be a button to search through the 'packages', and search for 'alsa'. if you dont have the latest version (there should be a table that lists your version and the latest version) then upgrade and test it with an mp3 or something, or reboot to hear the login sound.
if this doesnt help, then look on google or yahoo for 'linux driver [insert sound card name]' or something along those lines.. hopefully youll be able to find out. if so, it will almost certainly have instructions on how to install it. if you cant figure it out or none of the above helps, let us know, hopefully after testing afew things (and double checking the volumes are up, mute IS off, cables connected, and everything is how it should be!) then let us know.