Having done a small bit of research, I conclude that there is no voice of James Gustovski. Play the missions with him in, and try calling out something like injured or ready to fire. You will then realise this is nothing like the voice you hear in cutscenes. That is just a recording, so realisticly the only way to get his voice would be to either get someone who can immitate it, or get the guy who recorded it for BIS in the first place. If you wanted to find 'Gustovski's voice, you could try decoding the campaign, but you won't get the voice as in cutscenes.
-=GC=-Fungus1999