buildStream.js

var Promise = require('bluebird'); // or load bluebird some other way
var getSoundCloudUser = function(artist, params){
      var get = Promise.promisify(Soundcloud.get, Soundcloud)
      return get('/e1/users/' + artist + '/stream', params);
};

var stream = {
  tracks: [],
  playlists: []
};

function buildStream(group) {
  var artists = group.artists;
  var params = {
    limit: 5
  };

  Promise.map(artists, function(artistName){
    return getSoundCloudUser(artistName, params);
  })
  // extract tracks and playlists for each artist
  .map(function(items){
    var tracks = items.filter(function(x){ return (x.type === 'track') || (x.type === 'track_repost') });
    var playlists = items.filter(function(x){ return (item.type === 'playlist') || (item.type === 'playlist_repost') });
    return {tracks: tracks, playlists: playlists};
  })
  .then(function(sets){
    // get a flat list of playlists and tracks
    var tracks = sets.reduce(function(a, b){ return a.concat(b.tracks); }, []);
    var playlists = sets.reduce(function(a, b){ return a.concat(b.playlists); }, []);
    return {tracks: tracks, playlists: playlists};
  });
}

// usage:
buildStream(group).then(function(stream){
  // stream is in format: {tracks: [...], playlists: [...]}
}, function(error){
  // there was an error at some point (probably one of the Soundcloud.get calls)
});