OK, so here are the basic requirements, for a facial appearance in the mod i must have a Face map to place on a character, that requires 8 photo's of your head, 1 Front, 1 Back, 1 Top (hair), 1 Bottom(chin), 1 Left, 1 Right, 1 (3 Quarter Left), 1 (3 Quarter Right).
Great tut on this:(20 min long, prepare some tea or crack)
http://www.moddb.com/games/half-life-2/tut...e-mapping-videoFor Voiceovers you just need to be a good actor, meaning that the words you use must have enthusiasm. it's all scripted and i guess that if you volunteer I'll just hand you the lines i need in your voice and you take it from there.
PS. this stuff is not for the faint of heart, if you can't do something entirely please say so, so i can assign someone else to it.