How could I "mix and match" different sounds in order to make a new sentence? I know it's possible, I've seen new sounds in the Cinematic Mod. How could I do it?
As far as I'm aware of, there's no magical method to make it sound right, you're going to have to experiment; it comes down to timing and pitch. As for blending, select a the end of a sound, go to Effect > Utility > Fade Out. Then put the faded-out sound on top of another clip (you might need to Fade In the second one), and adjust the timing.
Are you talking about dialogue? Please don't do this, unless you're really lucky, emphasis will be way off and it will sound unnatural, unless of course you're making new lines for the G-Man, which would be the only acceptable use of this method.