I actually got really annoyed by the Rutger Hauer bits in Out of the Shadows! Just sounded odd to me.
it sort of made sense to me given how basic the computers on the Nostromo were, and how basic the escape pod/shuttle would be.
In pretty much the entire canon WY are meant to be a bunch of penny pinching criminals who only supply what is needed to get the job done at the lowest cost, whilst they might be willing to pay for a good voice synth for their android, the computers have a more basic/different unit that isn't as versatile.
It also ties in a bit with real life, when you consider that it's only relatively recently that hardware/software could replicate a wide range of voices reasonably realistically as opposed to having to tweak it at a hardware level or load specific samples into memory (anyone remember when your soundcard had it's own memory so you could load the samples to replicate midi sounds better iirc that was basically how many speech synthesizers of the time worked).
I remember how different the same tune could sound depending on which soundcard you used, so as a reason for Bishop sounding different it makes complete sense.
I went from an adlib compatible* to a SB16 compatible, or an Awe32 compatible, to a Live card complete with all the effects it could do in real time! then couldn't really tell the difference much with later cards.
*Around the time of Aliens your choice of soundcard massively impacted how your game sounded, right down to how many notes it could do at one, let alone how it coped with artificial speech.
