PROTIP: The X4 BGM folder for PC is structured the same as X5's. The only thing you need to watch for is the two Sigma boss themes are reversed.
As for modifying the voices, that requires some hex editing skill to change the in-game audio. The pre-fight boss quips are actually stored in the BGM folder, and are much easier to manage. However, only Colonel, Iris, Double, General, and Sigma's lines will play in the non-Japanese versions, regardless of whether or not the correct files for the 8 mavericks are in there.
I think the routines that call up the maverick lines are in the game's .exe, but I don't have the skills necessary to re-enable them. If I could, I'd add those back in, as well as the extended version of the intro from the Japanese version.
... Outside of the cut maverick lines, I wouldn't worry too much about trying to mod the game's audio to give the game a decent dub, there's probably someone
already working on that.