I just want to say "thank you" again for making such a dependable, feature-packed, fairly-priced, and downright awesome system/skill - I've had My Media for Alexa for close to a year now and have been very happy!
I am certain one of your most-requested features is the capability for multi-room music playback. I also realize that you all have done the best you can to bring us this feature, but that you are limited by the current Amazon APIs/the "AudioPlayer" interface in the Alexa skills kit is not designed to work with multi-room music or Alexa/speaker groups. It's really frustrating that Amazon has not made any headway here to make this a possibility.

(lol, I will admit that in times of extreme desperation, I will run from room to room, will open MyMedia skill on each of my Echo devices and play then pause the song I want certain spot on each device, and then will frantically run from room to room and will un-pause MyMedia at the same exact place on each of my 8 Echo devices in an attempt to have multi-room music but, yeah... this is not a viable solution, lol.
I do have a suggestion however - something I have toyed with and that does work and that I think the My Media for Alexa team could fairly easily implement into the existing skill/tool/server. The AudioPlayer interface in the Alexa skills kit does allow for playback of "live"/streaming media - this is how we are able to have multi-room music on other skills such as "TuneIn" radio, various podcasts, Flash Briefing skills, etc. I am not a developer, but I played around as best as I could and did manage to build my own personal demo Flash Briefing skill, setting the audio source to a live-streaming MP3 file on my own local server. I was then able to go to each of my Echo devices and open/play that "Flash Briefing" skill. When I went back to my server and played music with the output set to stream to that local MP3 file, I instantly had beautiful, synchronized music - playing together at the same time on all of my Echo devices throughout my home!
However, like I said, I am not a developer and barely knew what I was doing/was only able to make this happen using several "Flash Briefing" skill templates that I found online and picked-apart/merged together. My home-made multi-room audio skill worked for about an hour, but then my "Flash Briefing" 'episode' was old news and would no longer play.
With My Media for Alexa, we already have a dedicated local server hosting and playing our own media. Could you all maybe create a "virtual" local-playback device/output/stream/MP3 file on the My Media server that the user could then choose to select in My Media on any/each/all connected Echo devices? Perhaps this would require a second "My Media-Multi Room" playback skill? I know it would work! β This is very similar to how other personal-server multi-room music streaming services work, such as "Squeezebox,'" etc. Perhaps we wouldn't need a second "My Media Multi-Room" skill, but could just request at each of our Echo devices for My Media to "play the multi-room stream" or something similar, and My Media for Alexa would play that live-streaming MP3 file located on our local My Media server.
I will continue to fiddle around with my clunky skill when I have time and am happy to share anything that you might find helpful, but I am sure you all have the knowledge and awesome development skills to make this work!
I just thought it wouldn't hurt to ask/beg...
(running across my home trying to synchronize My Media playback on multiple Echos just isn't really cutting it, lol
Thanks again!
Richard