i was just wondering if someone could offer opinions on why in adventure games, voice actors who sound almost assuredly like native english speakers, mispronounce really common words or place emphasis on wrong parts of words or get the cadence horribly wrong?

Maybe some are not professional voice actors and were all the developer could afford.

Emphasis or cadence is affected by context -- so unless the actor understands what is happening in the game (rather than just reading the lines from the page with no context), they may use the wrong emphasis. A lot depends on whether someone who understands the script is directing the voiceovers.

they don't read from the script itself?

The actors may not actually be native English speakers. Very few adventure games are made in the US anymore.

most of the ones that come to mind started out as german releases. if they aren't then most have the accent down cold lol

Jadekitten -- yes, they are reading from a script, but that doesn't necessarily mean that they are reading (for instance) everything that all other characters are saying before and after their dialogs, or that they are cued into the circumstances surrounding the dialogs -- things that might influence how they should respond in terms of emphasis.

Some games have huge amounts of dialogs, and sometimes responses are applied to more than one situation.

For instance -- in Nancy Drew: Tomb of the Lost Queen, Nancy has a typical cheerful "good-by" dialog that ends a sequence, but sometimes the seuqence shouldn't end cheerfully -- sometimes she's been threatened or she's angry. That's the kind of situation that a director would be aware of and an actor might not be.

I don't mind too much if a translation is slightly awry or an inflection isn't quite on the right bit.
It's when a voice is toneless and bland and boring that I mind !!!!

I recently did some voice work for Ghostlady's upcoming game, Cliffhouse, and although I've acted in many plays over the years, I found it challenging to get the voices just right. I did read over the entire script several times, but without the benefit of working in a studio with the other actors there, it was difficult to know if my take on a line was the correct one. Also, considering all the different equipment used and the different acoustic environments in which the recordings are done, it's very difficult to get the same quality between different characters in a dialogue.