There are a few types of audio used by the game.
Most .Wav files use the Xbox Audio Codec. (xb-adpcm)
The music is either .wma or .ogg depending on the platform.
The easiest thing to change in game. Just encode audio as either (.wma for xbox or .ogg for PC) and rename as desired file and the game will play your audio.
Ambient Sound / Creature noises. .Met is basically a meta file for the .lug banks.
Script and regular Dialogue Entries. Composed as one sound bank with table listing at tail of file. Usually the same frequency, channel, codec.