this is a problem that we have had for a couple of days now, when accesing the api (with the proper client id) in order to get the track list for 2 artists Dj Tiesto and AVICII we get no results, even if they have tracks on the current website, it cannot be over limit use because both djs have "Pro Unlimited" accounts so they don't have that problem.
As you can see this link works perfectly http://api.soundcloud.com/users/aviciiofficial?client_id=CLIENT_ID
But this other one http://api.soundcloud.com/users/aviciiofficial/tracks?client_id=CLIENT_ID, gives us an empty array, it happens no mather the ip or the user that is accesing the url, and many others authors that are also very popular don't have that problem.