How to add audio / sound / music to your Neocities website!
Copy and paste this to your website: Change the youtube link with whatever you would like to play in the background. Make sure to not remove ?rel=0&autoplay=1, because this makes it possible to embed and autoplay. What we are doing here is making an iFrame tag that is 1x1 pixels big, so nobody can see it, but it will still play in the background though. Make sure to place it inside the body tag.
Another way to do this is to have an .mp3 uploaded somewhere on the internet, and then use HTML embed tags to place it on your page.
Note that I don't think Neocities allows uploading of mp3 files.