In my case this was because my sound settings were set to 5.1 (surround, with a center speaker), and the videos that I couldn't hear were mono (not stereo), and the mono sound was playing through a center speaker (only) that I don't actually have.
The solution: I opened my Sound Blaster control panel, switched it from 5.1 Surround (which must have been the default, because I only have a 2.1 speaker set) to Stereo, and now it all works.
Link