Avatars go in a a subfolder under the root Baldur's Gate directory by the name of Portraits. If you don't have this folder, then you need to create it. Portraits need to be in 24bit windows BMP format. The standard sizes are 110x170 for large portraits and 38x60 for small; I have no idea if you can use custom sizes or not, but remember that the game runs in a low resolution anyway.
Similarly, custom sounds go in a subfolder called Sounds, also found in the root Baldur's Gate folder.
Custom sounds are named according to the following format:
XXXXXXXa.wav : Battle Cry
XXXXXXXb.wav : Becoming Leader
XXXXXXXc.wav : Tired
XXXXXXXd.wav : Bored
XXXXXXXe.wav : Badly Wounded
XXXXXXXf.wav : Selected 1
XXXXXXXg.wav : Selected 2
XXXXXXXh.wav : Selected 3
XXXXXXXi.wav : Action Acknowledgement 1
XXXXXXXj.wav : Action Acknowledgement 2
XXXXXXXk.wav : Action Acknowledgement 3
XXXXXXXl.wav : Being Hit
XXXXXXXm.wav : Dying
XXXXXXXn.wav : In Forest
XXXXXXXo.wav : In City
XXXXXXXp.wav : In Dungeon
XXXXXXXq.wav : Daytime
XXXXXXXr.wav : Nighttime
XXXXXXXs.wav : Rare Selected 1
XXXXXXXt.wav : Rare Selected 2
XXXXXXXu.wav : Rare Selected 3
XXXXXXXv.wav : Rare Selected 4
XXXXXXXw.wav : Reaction to Party Member Death
You can find custom portraits and sound packs if you do a little google searching.
