libsound is from ALSA which was added to Slack9.1.
You will be dealing with a lot of dependencies. The easiest way to get Gnome 2.4 on Slackware 9.0 is to run the Dropline Gnome Installer (hopefully you have broadband otherwise this will take several days on dialup) They have taken care of the dependent programs and put together a real nice desktop.
Dropline Gnome
If you want to do it youself I would get the Slack 9.1 disks which have gnome 2.4 to start with.
Another option is to use use one of the slackware upgrade tools like swaret