I'm pretty sure it was streaming, I had the progress bar lit and moving in the mini-player at the top of the screen, and I also adjusted the volume (albeit clumsily) with the mini-player's volume control. Also, when the browser-crash message popped up (I was fullscren in a game ATM), the music stopped.

I also did some downloading of the files, and tried to play them while I DL'd the next one, but the second DL'ing file stopped midway through for some reason