If you were to try and play this game with audio in 15.04 you would need to actually symlink for the current libopenal.so.1.15.1 file to the ut2004/System directory replacing the default openal.so file that resides there.
Essentially running something similar to the following:
ln -s /usr/lib/x86_64-linux-gnu/libopenal.so.1.15.1 /home/itnet7/ut2004/System/openal.so
(Please make sure you adjust the path to reflect your installation, and username)
Once I created the symlink, I was able to launch UT2004 normally (without using aoss or padsp) and sound worked.
To install the game, I used my backup from many years ago, extracting the main UT2004 installation directory to my user account's home directory and upon launching it ran into some of the issues with libstdc++5 and libSDL mentioned here. I discovered that link after getting I was able to get the game running, and it helped me fix the sound. I'm posting it here in case it helps save someone else some time that might be looking for it as well.