My setup is as follows:
I use a single 7.1 USB soundcard over WASAPI and split it up into two zones. When I link the zones and start to play a local media file, everything is perfectly in sync.
As soon as I play an Internet Radio Station (directly) or Spotify (through the JRiver WDM driver) the zones are always out of sync. Link timing is not helping because sometimes zone 1 is ahead, sometimes zone 1 is behind zone 2.
As a dirty workaround I created a single zone which plays to both physical zones by routing the channels with the parametric equalizer. This is always perfectly in sync, so there is no hardware issue with my system. Actually I confirmed this behaviour on different systems with different soundcards (USB, internal, with ASIO, with WASAPI and with Direct Sound)
Could you please fix this syncing problem? One idea would be to dynamically merge all zones of one sound card. E.G. as soon as I link zone 1 and zone 2, you create an invisible zone1and2 which merges all DSP settings and the playback starts to zone1and2.