I already had the KLite codec pack installed, but I installed the HEVC codec from your link and it looks like that did the trick for playback.
But the buttons still don't work.
When I go to the settings for the controllers, the display shows nothing strange, and the list seems to indicate that the buttons are mapped correctly- but they don't respond to inputs. Strangely, the up and down on the Vive pad does seem to register for scrolling in menus, but left and right do not register.
I have not changed any bindings settings for any controller in any app or in Steam.
The only other change I made to any software that correlates to the approximate time frame in which the controls stopped working was I installed the SimpleVR video player (to see if the AV sync was off in that player as well.)
The behavior of the Vive wands in the SLR app seem to be identical to how the wands performed when the SLR app for Vive was first released, before the update. But that was before I built this PC, and everything did work on this PC when I initially installed SLR.