Very unlikely this is ULib's fault. To narrow down the possible causes we're going to need more info:
What sound are you trying to play? What exact command (command and any arguments) did you run to try to play the sound?
In what folder is that sound located on the server (Garry's Mod Server, not FastDL)?
In what folder is that sound located on FastDL?
Check your local Garry's Mod download folder (C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\download) and see if the sound file exists there.
I believe ULib only checks serverside to see if the sound exists, so the extra checks to verify that FastDL is working aren't necessary, but better to double check.
edit: ninja'd