There aren't different streams and there was nowhere on the site's FAQ mentioning bandwidth usage, but I dug this up on their forums, which suggests listening may take 60MB per hour. That sounds like rather a lot. If I listen for an hour a day that's... 1.8GB per month? I get 25, but I don't know how much of that is being used on playing on Battlenet and watching Youtube or on all the things the boy gets up to.