Game has installed correctly but only has sound effects and no audio Dialouge.

I don't know the fix but it doesn't surprise me. The game's new audio engine has always been a huge buggy mess that's a big step back from the 15 year older originals in many ways:-
https://www.youtube.com/watch?v=2zVGRAE9NBA
https://www.youtube.com/watch?v=2I5q8Ae375Q

You don’t appear to own any games here?

Anyways, I agree with BrianSim, it is a mess. Was watching someone else play it, sound is all over the place, bugs galore (guards spinning on the spot for instance). I would highly recommend to look at the original games, or The Dark Mod which is a fan engine based on the doom 3 engine with loads of fan maps:
https://www.thedarkmod.com/main/

Could it be this? https://www.pcgamingwiki.com/wiki/Thief#Issues_fixed

If not then your best bet is probably to contact Support. You could ask whether anyone at https://www.ttlg.com/forums/forumdisplay.php?f=170 knows anything but the game's always had a reputation for broken audio (apparently it was even worse pre-patches).