Google Play Music
Same issue with version 1.0.435.42 on OS X 10.11.4.
A had some success when I had added '[*.]play.google.com' to add-on exception list (vivaldi://chrome/settings/content). But it works only for one song.
Same issue, 1.1.453.6 on Windows 10 x64, and the yanick76's workaround didn't work for me. It's a deal breaker for me, I hope you guys can fix it.
Same issue with stable 1.0.435.42 (32-bit).
Replicated with Windows 7 Enterprise SP1 (64-bit) and Windows 10 Pro (64-bit).
After some testing, it seems like any songs that were ever uploaded or purchased via Google Play will work, but any songs being streamed via the Google Play Music subscription service will not work.
Not sure why it works this way, but hopefully that information is helpful.
EDIT: Just wanted to add that I tested with 1.1.453.45 (32-bit) and have the same issue. This was on Windows 7, but I will test with Windows 10 when I get home.
I'm seeing this with Vivaldi version 1.2.490.39 () (32-bit) running on Windows 10. As stated above, it's fine playing uploaded music, but won;t play anything from the streaming service.
I use earbits in the web panel http://www.earbits.com/#genres/