Is it possible to change character's portraits and sounds with after you have started a game, possibly with SK or some other method? Secondly anyone know a good site with custom portraits and sounds?
Yes, it is possible. You just go to the recod page and press customize. Then you choose appearance and there you go. I really can't remember any sites, but you'll find many good ones just by searching from Altavista with 'portraits'. Happy hunting!
"Ah well, you'll learn. This wizard business is complicated." -Zeddicus Z'ul Zorander
AS Moonmaid said, in the game, go to the Character Record page - the button with the head on it. Click on Customize, you'll see a list of things you can change: sound, colors, scripts etc. This only works for your character, not NPCs.