1+3) Windom folder\pilot, there you should have the 4 pilots the game has by default, there you can pretty much see what you'll need.
a .png for the pilot's face (200x150 rez-wise, or any rez of that format)
a small txt with the pilot's name
and severall sound files, which you'll need to name to mirror the ones in the default versions, your choice as far what you pic.
as far as voices go, it really depends on what pilot you want to make, easiest way is to rip sounds from a PS2 game (if available), use ps sound (there should be a link to it in the "why not listen to your favourite characters" topic). only other way I know of is to rip sounds from video/audio files that you have of that character, arm yourself with a lot of patience if this is the case. Once you're done ripping and naming the sounds accordingly, encode them in OGG and you're set to go.
2) Much easier, simply convert into .ogg any BGMs you want, then put then in the windom folder\sound\my bgm, They will be available in online games. If you want to change a stage's bgm, then rename the .ogg accordingly and put it in the windom folder\sound.
fuse - stage 1
br - stage 2
gg - stage 3