I am pretty sure I know exactly what it sounds like. I had a talking alarm clock from the same era and everything that made sound electronically back then did so by using the same chips with the same samples as everything else. (Including pocket 'trick noise makers' and car alarms - those two categories even had some overlapping samples if I remember right)
Museum. Museum! i don't doubt you (especially given how special they stood it up), but they sell these at the convenience store down the street from me. it's not a "clear voice".
The museum display this watch inside a timeline to show you the evolution. It's actually neat of them to put cheap watches in the general exhibition. You have a broad range from this cheap watch to an old disused atomic clock.