We know that the upcoming Metal Gear Solid HD Collection is going to look pretty with its visual upgrades, but it may also get an upgrade in the sound department. Check out this recent tweet from Hideo Kojima:
Re-recording for “MGS HD (MGS3)” in progress today, and the voice actors from MGS are here from the morning.
Of course, that could mean anything. This mention of voice work is probably Japanese anyway. There's no confirmation that anything is being changed beyond the third game, mind you. And it's not that any of the games need new audio, either.