My son added back in the "sound card" that was originally "there" in the "Playback devices" list when right clicked on the little Windows sound icon. Previously, when he disabled the Vizio's "sound card" in Windows - it disappeared rather than just graying out like other things do when you disable them. Thank you Microsoft. So anyway, he right clicked on the Realtek thing and that dropped down a menu that allowed him to check-mark to display the disabled sound items (the Vizio thing that had gone missing). And then he made Vizio the default (in the list) and the sound started working again. No clue why that worked or why it had previously quit working.
No sound when I connect my HP Pavilion 300-020 mini PC running W10 to Vizio TV SV472XVT with HDMI cord. Worked OK last week. Tried 2 different HDMI ports on TV. Tried 2 different HDMI cables. It works OK on my other Vizio TV that seems to have Intel sound card. My Sony Blu-ray player works perfectly when connected to same HDMI ports. My laptop works perfectly also on same HDMI port. Vizio support chat had me unplug the TV and re-do everything but no joy and Vizio support says the TV diagnostics are perfect. On the TV screen's PC display, the volume icon has a red X, but Device Manager looks perfect. I tried to let W10 find and update the Audio driver but Windows said I already have the latest driver. All signs seem to point to a problem with Windows Audio on the 300-020, or at least when hooked to the Vizio TV. 300-020 uses wireless keyboard & mouse - it has no display screen of its own.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic