Not sure that you could easily. The files are in .dat and .mdx format, so you could probably alter them, but you might need some information that only the dev team/audio packager has.
Or, of course, you could see if there was a way such as putting in paths to audio files, but I'd sincerely doubt that, as HW does not seem to be terribly mod supportive.

There was a cheat to change all the voices to those with a Liverpudlian (Scouser) accent.
"There's a big boat here, lets nick it!"
Do the cheats work with the GOG version, I haven't tried yet?

you australian?
Cos i also thought the voice acting was great... except for the terrible attempt at an english accent... the aussie one didn't bother me.
Wonder what other nationalities thought of the other voices?

Then I have to say "Man the voice acting was great..except for a terrible attempt at an AMERICAN accent!"
heh. Actually I can say that some of the voice acting in the beginning intro was..interesting.

How can the English accent be horrible if the game is made in England and everything? :D

The game has 3 famous UK TV actors featured in it. Well, famous in the later 70's and early 80's.
Tom Baker did the narration/voiceover intro. ( the fourth Doctor in Dr Who)
Paul Darrow and Glynnis Barber ( they of Blakes 7 fame, they played Avon and Soolin respectively) provided the majority of the in game voices. SO it's quite possible Glynis Barber who I think is welsh, couldn't carry off an aussie accent.
Anyway, bit of trivia for you there :)
Blakes 7 was excellent by the way, despite some shaky sets, the occasional bad episode and some awful acting by the coloured actress who played Dayna ( god she was shite), but other that that, it had lots of excellent actors pass through its doors. And it did not end happily. And unlike so many TV shows theses days, it did end, as in all story lines bought to a close etc, and the ending was very final.

he only time voices really really annoyed me and practically stopped em playing the game was Rome Total War with it's Australian accents - I mean, in Roman times, Australia hadn't even been discovered!
almost any European accent would have been better, but just because it was made in Australia they put in Australian accented voices instead of going out and finding Italian-Australians or even English-Australians that are everywhere in that country!