Subsonic is basically abandonware; it isn't open-source and there is no development or active support by the owner. You might consider moving off to a newer implementation. Both below suggestions run on multiple platforms, I'm a Windows guy so some of my supplemental info isn't as useful to non-Windows users.
If you're looking for a very lateral move, consider Airsonic Advanced
https://github.com/airsonic-advanced/airsonic-advanced - It is in active development with frequent snapshot updates
https://github.com/airsonic-advanced/airsonic-advanced/releases . Same feature set as Subsonic (API, Sonos, etc.) with updated code. As it is open source, you also get almost all the features Subsonic Premium gives you, but for free (see my last line). Minimal effort for installation (latest Java installed, then a command line shortcut to the war file - upgrades even easier with just a fast war file change).
If you're really more API focused, and looking just for a music streaming service (app over browser), you might consider moving off the Subsonic family of servers altogether. Check out Navidrome
https://www.navidrome.org/ . That product is primarily to supply the API with a completely new back-end, and refocuses to just support audio (no video, podcast, internet radio, etc.). There is not yet built in support for Sonos, but you can find easy linking with something like Bonob
https://github.com/simojenki/bonob. As with Airsonic Advanced, no subscription or fee to access the API. Navidrome does have a simplified web UI if desired.
Both products will need you to come up with your own DDNS solution for external access. Both support running as a service with something like NSSM
https://nssm.cc/, and IIS works great as a reverse proxy if you wish to run them as SSL.