I have had this same problem (*.mp3,*.wav) a few times over the years on my XP system with Audition 3.0 but so few and far between I always forget WHAT fixes it which bought me to this thread tonight. I tried a few suggestions other than messing with the registry and suddenly remembered what has fixed it before and I SHUT DOWN MY COMPUTER and restart it and... like MAGIC, the mp3 association is back to what it is suppoosed to be!
Don't know if this will work for anyone else but just wanted to offer this option for those of us that will spend HOURS looking for answers before trying (as my son the IT guy says) the OBVIOUS FIRST STEP! LOL
Good Luck
