Following just a few examples from other people who have Webamp on their websites, you just should make the "initialTracks" array organized like this:
  initialTracks: [
   { metaData: { artist: "The Boy is Mine", title: "Brandy & Monica", }, url: "https://sadhost.neocities.org/music/Boy.mp3", }, // music 1
   { metaData: { artist: "blah", title: "Music 2", }, url: "https://test.test/music/music2.mp3", }, // music 2
   { metaData: { artist: "blah", title: "Music 3", }, url: "https://test.test/music/music3.mp3", }, // music 3
   { metaData: { artist: "blah", title: "Music 4", }, url: "https://test.test/music/music4.mp3", }, // music 4
   // etc
          ],
Replace the artist, title and URLs to the music info you want to add.
I don't know if this is what you were looking for but I hope it helps.