I'm not going to try to claim I know more about audio recording than I do (which isn't much). But I believe the studios invest in those recording booths for a reason. When all the sound is recorded in the same place you control the acoustics, and everything comes out at a consistent volume/tone/pitch. When you start pulling pre-recorded lines from other sources you have no idea what the conditions were for those recordings. I'd imagine MLB broadcast booths have very different acoustics from SCEA's studio recording booth.
Anyway, what I'm getting at is that the lines aren't going to sound consistent if you pull them from different sources, even if it's the same person's voice. And trying to normalize audio from a bunch of different sources would probably require far more development time than it would be worth.
And that's just for basic PBP, describing events as they happen on the field. I don't even want to think about splicing in player names in an environment like that.