This reads like a SoftBank slide deck.

Reading this makes me yearn for the ability to downvote comments

Here is my prediction. If the speed of light were 5 km/s, it would be slower than the speed of sound; consequently, the sound would reach me much faster than the visual image. For instance, if I were surrounded by ten people and one of them clapped, I would hear the sound first and only realize who had clapped quite a while later.

Sound is a wave that travels through some medium, which also cannot be faster than light. You can't have FTL information transfer.

Actually, all the [physics stuff] that allows sound to propagate would also be slowed down, to <5 km/h.

More realistically - the atmosphere would not be gaseous (everybody dies), the sun would be a black hole larger than the earth's orbit (everybody dies), and quite a few other problems.

You don’t have to go that far: Earth itself would be a black hole

Didn't even think of this, but yes: 5km/h ~ 1m/s, which is 1/10^8 the real c. This multiplies the sun's Schwarzschild radius (normally ca. 3km according to Wikipedia) by 10^16, making it 310^13 km, with 1AU = 15010^6 km.

It's yet another xkcd-level apocalypse.

Sadly, Google's AI search can find no hint of St. Randall working on a What If? 3 book.

(But What If he's sworn it to secrecy?;)

if i be honest. textbooks are pointless nowadays. AI can make any stats into a short note and it's only going to get worse. on top of that youtube(not just youtube) has ton of stat videos where people don't need a textbook to read.