Apparently they decided to not bother with the expense of voiceovers for the english release (although if you dig into the script files, you'll see they reference voice files which are actually there, only that they are all filled with several seconds of silence).

Someone released a voicepack compilation of voices from SW1 and SW2, click here and read the second post (the one by Gui DURANDAL). Let me know if the link is down because I've saved it. I've also created two replacement voices for the hero in the "Star Wolves Talkie?" thread, although you could simply rename the files and move them to a different pilot's folder.