Installing latest K-Lite with Icaros 2.3.0 b5 stopped DOpus from crashing on me when I click on a FLAC!
If I have both the Viewer and Meta pane open, is there a FLAC-capable player display in the Viewer pane? It's great having the tags show in the Meta pane!
If you have the codec + splitter installed to make FLAC work in Windows Media Player, you should be able to assign .flac to the WMP preview handler (does not auto-play) or ActiveX Control (does auto-play) via the AcitveX + Preview + Office + Web plugin (in Opus: Preferences / Viewer / Viewer Plugins).
K-Lite is installed and .flac plays fine in Windows Media Player. I added .flac to the ActiveX Control as you described. It still just displays the thumbnail of the embedded album cover in the Viewer window. I tried enabling the Windows Media Player plugin, and it plays the file fine, but as you say it does not auto-play.
Any idea how to get the ActiveX Control to play the file rather than display the embedded album cover?