I have the Sony SRS-X5 bluetooth speakers and was having the same issue. I tried many methods, but I ultimately found one that worked for me.
*Firstly, go to Bluetooth menu icon and turn off your bluetooth, then follow the instructions below:
1. Go to Finder
2. Under menu item "Go," scroll to bottom of options to "Go To Folder"
3. Type in: /Library/Preferences/
4. Find the .plist that is associated with bluetooth. It should be a lonely file called: com.apple.Bluetooth.plist
5. Select that .plist and trash it - I like the command button + delete option
6. Restart your computer
This should give a refresh on your bluetooth devices. I have a bluetooth mouse and keyboard that both worked normally, but my external speaker would show a connection, but not push output sound to the speaker. It worked on all my iOS devices as well as older versions of the OS, but Sierra messed it up (currently 10.12.6).
This did solve my problem ultimately. However, I had to try it twice, for whatever reason. It did not work the first time I tried it. Now it works normally.
I hope this works for you. Good luck. Cheers.