if you're like me, you probably can't stand the unreliability of youtube audio players. i'm old enough to remember the joy of putting autoplay music on my myspace page, so i've been adamant about autoplay music for my tumblr, and now my neocities page.
copy the code above. to toggle whether you want it to automatically play your song, or loop your song, delete autoplay="" or loop="" respectively. the code is set to autoplay and loop by default.
replace "Your Link Here" with a direct link to an MP3 file. i use dropbox for mine - if you choose this, replace "https://dropbox.com" with "https://dl.dropboxusercontent.com", or else it won't work!