It is possible to change the sound scheme in Ubuntu 10.04 (10.10 may be the same, I'm not sure). It's a bit fiddly but it does work well. BTW, I disagree with pbpersson above, I would leave the default scheme as it is, you don't need to touch it.
The trick is to follow the default Ubuntu sound directory format exactly including the names of the sound files (the filenames need to be the same as the default scheme but of course the sounds can be completely different).
I'll use as an example a sound system I set up recently called "Ethereal" (just replace "Ethereal" with the name of your scheme): First create the directory: "/usr/share/sounds/ethereal".
Now copy the index.theme file from the default Ubuntu sound scheme directory into the ethereal directory.
Edit the copy of the index.theme file and make the appropriate change (e.g. replace "Name=Ubuntu" to "Name=Ethereal". That's all you need to change in this file, so save it.
Next, create another directory "/usr/share/sounds/ethereal/stereo". This stereo directory will contain the actual sound files but they must match the names of the files in the Ubuntu scheme.
Now go to System, Preferences, Sound. There in the box beside "Sound theme:" click on the down arrow and you should see your new scheme name there, choose it and your sound scheme will be changed.
I've made several schemes now, it's really a lot easier to do this than my attempt to describe it above. I've downloaded some schemes from
Ubuntu-Art.org sound schemes. Some of these sound schemes are formatted as above but some are quite different. Some have installation scripts to process their own way of changing the sound scheme which might differ from the above default scheme design.