There is a bit of an issue with audio in PoP:SoT, as I experienced both of the problems you're having. It seems like the EAX, 3D Audio Acceleration and Audio Virtualization doesn't work so well, and often drops off sounds very quickly, even though they are close to you. The trick is to actually get the camera close to the audio source rather than the Prince (this is for in-game dialogue). When things are being said, I usually zoom in as much as possible so I can hear.
Anyway the solution to this which I found, for both in-game and cut scenes, was to turn off all the fancy EAX stuff etc (these are the settings I use) as it then will pipe through your left and right speakers.
Essentially this is dependant upon what set-up you have, so 2.1, 5.1 etc. If you just have a pair of desktop speakers already, you're not going to notice much difference, but with 5.1 for example, I could make out that generally speaking the voices were a little louder and clearer once I turned off the surround sound stuff.
The last solution is, unfortunately, headphones. This makes everything pretty much audible and easy to hear.
Don't forget to go to your Control Panel in Vista, open up 'Sound' and check that you have changed your speaker setup to "Stereo" as this might also help to make the voices louder when combined with what I have described above.
Have a play around, as I don't know what hardware you have, to find the best solution within these different settings. Good luck!