I don't believe that there is a UI for changing those sounds, so it's not something that's easy to do.
That doesn't mean it's impossible: if rooted (quite liked on Cyanogenmod!), you can replace /system/media/audio/ui/KeypressStandard.ogg with another .ogg audio file of your choice. I'd make a backup of the original (rename to "KeybpressStandard.ogg.old" for instance) just in case.