This first half of the post is just extra info for your consideration in the future, you can skip to the second half if you want. Only Blizzard sounds have numbers. PlaySound accepts sound IDs, which are assigned numbers that sometimes uses multiple sounds. For example, a character's attack sound ID usually has different sounds assigned to it which play randomly. PlaySoundFile accepts file IDs, which are tracking numbers for all files in the game install, not just sounds. Each one of these numbers always has only one file, and if it's not a sound, nothing happens if you use it in PlaySoundFile.
For example, the sound ID I used is 2304, which is a set of three mechanical wound sounds:
Those three sounds can be located using wow.tools:
File IDs 562730, 562733, and 562738.
In your case, all files provided by addons aren't a part of the game install, so they have no file IDs assigned to them. Just plug in the file path of the addon file into PlaySoundFile:
PlaySoundFile("Interface\\AddOns\\ShadedMedia_Causeses\\Spellsteal.ogg")
File paths have to be absolute. The root of the path system is where Wow.exe is. You can even put a file in the main folder and play it like this:
PlaySoundFile("Spellsteal.ogg")
This will play the sound using the default SFX channel. If you have that channel low or have sound effects disabled, use the Master channel:
PlaySoundFile("Interface\\AddOns\\ShadedMedia_Causeses\\Spellsteal.ogg","Master")
The five channels are Master, SFX, Music, Ambience, and Dialog.
Currently, if you want to add a sound file, you need to make sure it is at that location when the game client starts. In Shadowlands, they changed this behavior to allow files to be recognized when the UI loads, which is when you login to a character or use the /reload command.