It is very easy to add a custom sound to your Tesla.
Search for your custom sound and click on the download Icon to download the file.
Rename the downloaded FIle to "LockChime.wav". Please notice the capital L & C.
Upload the renamed file to your USB Stick in your Tesla. The file should be copied to the root folder. Insert the USB Stick back to your Tesla.
On your Tesla go to "Entertainment" -> "Beatbox" -> "Lock Sounds". Enable Lock Sounds and select "USB" from the dropdown.
Lock your car and you should hear your new custom Tesla lock sound.