ok, for Echo / Alexa to work, a developer would need to do quite a bit.
https://developer.amazon.com/public/sol ... our-systemNote all that "oauth" stuff: Subsonic has none of that. It has no token-exchange authentication at all. As such, a developer would need to create an app (probably best to host it on Amazon AWS/S3) that would generate oauth tokens and use them to wrap the Subsonic server credentials. Those who use that service would need to trust that the author of that app doesn't do anything malicious with the logins they effectively have been granted, because there is no way that information can be used without being permanently stored by that app.
Granted, if a future release of Subsonic used oauth instead of its current (relatively insecure by comparison) mechanism, that would make parts of this much easier.
(There are reasons I try to make it very clear that neither SubFire nor SubFireRadio are sending any communications to my own servers, nor even do I run google analytics on it. Anybody who uses my apps? I have no idea where their servers are or how to log into them: none of that information ever gets to me.)
Only after the oauth wrapper and account linkage is done can the developer start writing alexa-skills for playing (and pause, skip, back), as wrappers around the Subsonic API that would then send the final music-playing URLs back to the device. In addition, of course, would be the need to figure out how the device can recognize when a song has finished in order to know when to start playing the next song in the queue.
There's also the issue of whether or not the Echo and Alexa systems would accept self-signed certs for HTTPS. The Fire Stick does not, in any way, and I can't override that (in spite of openly stating that, I still have 3 1-star reviews solely on that topic). The Fire TV and Tablets are very difficult to add a home-cert to. My guess is that Echo would not support it, so that's another thing that would disappoint many.
It isn't technically impossible, and I'm fascinated by the idea, but it is a LOT of work.