I'm assuming the voices are hiding in the USRDIR area of the image as this is where the meat of the game lies:
FMV - This folder contains the SEGA logo as well as the Backbone intro as seen at the beginning of the game, as .pmf files. I have no need for these but maybe I can find a way to convert them in the future for archival purposes if anyone is interested.
FONTS - This contains three .fnt files. Basically just the fonts used for in-game text.
MODULES - Everything in here is a .prx file. They're all extremely small in size and I assume they have something to do with the coding of the game.
SOUNDS - This folder doesn't contain anything but another folder, called MUSIC.
---MUSIC - These are all .at3 files and they CAN be renamed if you'd like to change where certain music plays. I haven't tried taking .at3 files from other games yet but that would certainly be an interesting (if a little trivial) little project.
STREAMS - This is where things get really interesting. Everything here is a .str file. If you switch them around, you can switch which characters are which. For example, I renamed Knuckles' two .str files as well as Sonic's and they switched places. Putting Shadow against "Knuckles" in the challenge mode caused him to race what looked and sounded like Sonic. However, things got really interesting when I played as "Knuckles" against "Sonic". The camera actually focused on the actual Knuckles as player 1, but I had control of player 2, so something in the game's programming still referenced the characters as they should be. I'm sorry if the way I worded this made no sense.
TEXT This contains a .txt file containing all the text in the game. I changed Takashi Iizuka's name to my own and upon playing the game, all the text was gone and the game froze before a level would load. What gets really weird is that when I changed it back to Iizuka, these problems still existed, so it seems any attempt to modify the text, even if reversed, will destroy the game.
TEXTURES - Nothing but UI...
---UI - Nothing but LOAD_SCREENS...
------LOAD_SCREENS - This has a bunch of .bmp files. They're all the load screens of the game.
Since Sonic's and Knuckles' voices also changed when I switched the .str files around, I'm assuming the voice files are encoded into there. Does anyone know how to extract these?
Sonic Rivals 2 is very similarily-built to this game, so once I figure this out I'm sure Rivals 2 will be a cinch.