Audio and video won't play when using a Multi-Output Device: ...the Built-in Output (MacBook Speakers or similar) must be enabled and listed as the top device in the Multi-Output. If BlackHole is listed first simply uncheck and recheck [...] to change the order.

HEYYYY this should be on a banner in 100-pt type, not buried at the bottom of a troubleshooting page! (BlackHole is a freebie you might find handy if you can't afford Panic's audio apps...)