There are a few options:

1. Store the absolute or relative path to your mp3 file in your json file. This will have the risk that the path is incorrect, is someone copies only the json file, or renames the mp3 file. To do this just add a string musicPath field to your class.
2. Store the mp3 file inside your json as a base64 encoded string. This will make your json file harder to read, and it will waste some space due to the inefficient encoding.
3. Store your json and mp3 file inside a zip-archive. Store the entry name of your mp3 file inside your json file. This ensures that the user only sees a single file, but makes it a bit more cumbersome to create or edit files by hand. You can mark the mp3 file as "store" to avoid the compression overhead.

In all cases you will need to know how read and use the stream representing the mp3 data.

For music NFTs, I'd include your .mp3 in the animation_url per Metaplex docs.

I'd probably add these song genre, posting date, etc. as attributes, e.g.:

    "attributes": [
        {
            "trait_type": "Genre",
            "value": "YOUR_GENRE"
        },
        {
            "trait_type": "Artist",
            "value": "YOUR_NAME"
        },
        {
            "trait_type": "Posting Date",
            "value": "2023-02-16"
        },
        {
            "trait_type": "License",
            "value": "Exclusive"
        }
    ],

And again in your files:

    "properties": {
        "files": [
            {
                "type": "audio/mp3",
                "uri": "YOUR_MP3"
            },
            {
                "type": "image/png",
                "uri": "URI_OF_A_COMPLIMENTARY_IMG"
            }
        ],
        "category": "audio",

    },

And yes, external_url is your website.

fully metadata would look something like this:

{
    "name": "YOURSONG_NAME",
    "symbol": "SONG",
    "description": "some description",
    "seller_fee_basis_points": 100,
    "external_url": "https://YOUR_WEBSITE",
    "attributes": [
        {
            "trait_type": "Genre",
            "value": "YOUR_GENRE"
        },
        {
            "trait_type": "Artist",
            "value": "YOUR_NAME"
        },
        {
            "trait_type": "Posting Date",
            "value": "2023-02-16"
        },
        {
            "trait_type": "License",
            "value": "Exclusive"
        }
    ],
    "animation_url": "URI_TO_YOUR_MP3",
    "image": "IMG_EG_ALBUM_ART",
    "properties": {
        "files": [
            {
                "type": "audio/mp3",
                "uri": "YOUR_MP3"
            },
            {
                "type": "image/png",
                "uri": "URI_OF_A_COMPLIMENTARY_IMG"
            }
        ],
        "category": "audio"
    }
}

FWIW, I don't think there's anything special here in the metadata re: programmable vs standard NFT. Examples of both here.