My headcanon is that Number One is the in-universe voice actor for ship computers in Starfleet. Once they made the computer model of her voice, it remained in use all the way to the TNG era.
I doubt Lwaxana Troi would have been interested in sitting still for a hundred hours of voice recording.
At some point her face starts showing up on recruitment posters, so maybe? On the other hand, we only saw a poster like that 100 years after her time, so maybe not.
Alternatively, she leant her voice specifically to the Enterprise computer, as seen in TOS. Then years later, when it was decided to standardize voices across the fleet, they just went with what was used on the Enterprise. Because the Enterprise always seems to be the favourite of the fleet.
