Trains in London in 1992 had announcements using recorded voice clips

BART did too. I think the announcements date back to the early days of BART. The computerized text-to-speech didn't come around until 2000 and only cover train arrivals.

  Perhaps it sounded more futuristic than plain recordings?

BART's always gone for style over substance, so yeah that probably played a part in it. There's a small chance that text-to-speech was cheaper than paying a human.

In San Francisco, Muni paid a Texan to record stop announcements for their buses. I've absolutely no idea how this ended up being the case but she absolutely massacred the pronunciation of a few (mostly Spanish) words.