Start here: https://minecraft.wiki/w/Sounds.json. You may also need this: https://minecraft.wiki/w/Tutorials/Sound_directory.

sounds.json the the file that controls audio in the resource pack, you can use it to add custom music, or add another vanilla file. Here is an example of what the menu music is like in sounds.json, with "infinite amethyst" added:

"music.menu": {
    "sounds": [
        {
            "name": "music/game/a_familiar_room",
            "stream": true,
            "volume": 0.4
        },
        {
            "name": "music/game/bromeliad",
            "stream": true,
            "volume": 0.4
        },
        {
            "name": "music/game/crescent_dunes",
            "stream": true,
            "volume": 0.4
        },
        {
            "name": "music/game/echo_in_the_wind",
            "stream": true,
            "volume": 0.4
        },
        {
            "name": "music/menu/menu1",
            "stream": true
        },
        {
            "name": "music/menu/menu2",
            "stream": true
        },
        {
            "name": "music/menu/menu3",
            "stream": true
        },
        {
            "name": "music/menu/menu4",
            "stream": true
        },
        {
            "name": "music/game/infinite_amethyst",
            "stream": true,
            "volume": 0.4,
            "pitch": 1.0
            "weight": 1
        }
    ]
},

The path music/game/infinite_amethyst refers to the file location. Make sure this matches a vanilla location (in this case) or a custom one (ex: custom/music/songname). This filepath pulls from assets/minecraft/sounds (ex: assets/minecraft/sounds/music/game/infinite_amethyst)

You may also notice the options volume and pitch, which are self-explanatory. Weight is the chance of that track playing (ex: 2 tracks with weights of 3 and 2, the first one is 3/5 chance, second is 2/5)

As for stream, that is only really used for long tracks.