USAGE:
["_func", [["playSound", "soundTest.ogg", 10, false]]],

HOW TO USE:
Create a "sounds" folder in your custom guard folder if you haven't already. It's pretty much works the same way as playSound/playMusic, just the addition of adding ".ogg" to play local sounds. But I'll recap:

To play any of your sounds, ALWAYS include the ".ogg" at the end of the sound string so it can load sounds inside the "sounds" folder. For music (playMusic), insert it in the "songs" folder instead.
["_func", [["playMusic", "musBGMusic.ogg", 0.5]]],

To stop a custom sound, ALWAYS use "extSFX":
["_func", [["stopSound", "extSFX"]]],
If you use playMusic, use "extMusic" instead:
["_func", [["stopSound", "extMusic"]]],
Playing another custom sound without using stopSound will stop the previous custom sound to play the current one.

IF YOU ALREADY HAVE A MODIFIED data.win FILE:
GIF of the steps below: https://i.ibb.co/9m4fBqJy/jk-Di-Bl-IQMw.gif
1. Open up UndertaleModTool
2. Open up the modded data.win file.
3. Navigate to Code > gml_GlobalScript_CustomGuardsPlus
4. Copy all the contents of the txt file of the same name "gml_GlobalScript_CustomGuardsPlus.txt" and paste it into the code.
5. Save the data.win file! (Backup your old modded file first though!)
6. That's it! Run the game and have fun!