Here are a couple of sample scripts that might help you accomplish your goal. They require the scripting addition (also known as osaxen)
"Jon's Commands".
For my example, I created a folder named "Sounds Folder" on the startup disk and copied all the sound files from the System to it. Feel free to tweak as you see necessary.
-- Script 1 --
set sound_folder to (path to startup disk as string) & "Sounds Folder"
set sound_list to list folder sound_folder
set sound_count to number of items in sound_list
set random_number to random number from 1 to sound_count
set my_sound to item random_number of sound_list
play sound my_sound
-- End Script 1 --
-- Script 2 --
set sound_folder to (path to startup disk as string) & "Sounds Folder"
set sound_list to list folder sound_folder
set my_sound to some item of sound_list
play sound my_sound
-- End Script 2 --