Good question.  I had forgotten about this change.

The final argument of Audio::startCollisionSound() is a boolean variable that says whether to flash the the screen or not.  While I was testing sliding my avatar around the voxel terrain my screen would often suddenly go dark, sometimes pitch black and sometimes only dark gray, and stay that way.  Sometimes I could collide with the voxels again and it would fix itself, but more often than not the screen would be so dark I couldn't navigate well.  Logging out and back in would fix it.

Turns out there is a bug in this screen flash that sometimes causes the flash to kick in and get stuck.  I started to wander down the rabbit hole of fixing the problem but caught myself and instead disabled the flash for these collisions so I could make progress on debugging the actual collisions.  Then I forgot to re-enable it.

I'm open to suggestions about what to do.  I'm reluctant to just re-enable the flash, but if we do want it re-enabled then I should fix the flash bug now rather than later.