I have seen that some people got a little frustrated about when they select a police officer or any player model, he would still speak like Niko (i.e. not the model's own voice), either when driving into something, getting hit, or making him talk with the Numpad in LCPDFR.
So to fix this:
1) Select your player model in the trainer.
2) Go to page 1 of model spawning and click "Speech Menu".
3) Go down to where it says "Play random speech" and press it once. (He will say something random!)
And that's it. He should now speak how he is supposed to and you can carry on listening to those strange speeches!
Hope this helped.