N64 Sound Tool does this. But its algorithm for encoding to VADPCM isn't perfect.
edited 8:04 PM EST January 15, 2012
Algorithm by SubDrag at 9:31 PM EST on January 15, 2012
Calculating predictors is unknown so I just have in latest version 8 hardcoded predictor sets where I pick best result. I'd love for someone to figure out algorithm to calculate em. Note most games can import, but a couple can't (EAD games, compressed games, Mario kart style) but prob 90% can. Only Goldeneye supports raw import...its odd. All others vadpcm only.