Is there a way to upgrade text to speech voices?

I'm working on a course and we have a lot of audio.

I've used the British female voice for the text/speech.  There's been a mixed reaction to her voice, and I was wondering if anyone knew a way to purchase or upgrade other voices that are better and use that for creating audio?

Even if it's outside of Storyline, I'm assuming I could probably use a voice package that was better and create the audio files then drop them into Storyline?

I don't think the voice is that bad, but we've had a range of reaction.