Ah, that would be it! I used AudioStreamPlayer2D. I will be sure to use the proper AudioStreamPlayer in the future. Thanks so much! I think I got so used to picking 2D nodes that when I saw the 2D, I thought, "That must be specifically for 2D games." haha
The other audio problem I couldn't figure out was how to only have one copy of a sound play even when multiple instances were calling it. I may go back and clean up the code a bunch so I can practice knowing how to solve these issues in the future.