Even if you were to generate and install certificates and enable HTTPS, a system that old will not have the new ciphers and TLS versions - Browsers will reject it anyway.
Like the others, I'd recommend moving to another system. But if you want to keep it for whatever reason, you can put it behind a firewall and reverse proxy like nginx. Then you could translate the plain HTTP back-end to modern TLS.
Edit: This is not an easy task for a semi-technical home user and you will almost certainly mess it up. If you go anywhere near that server with an ethernet jack or a WiFi signal your network is basically guaranteed to be hacked and all your data stolen, bank accounts raided, credit ruined, and the next time you reboot your PCs they will all have a screen demanding payment in BTC to get your files back. For a music server it's not worth it.