Huh? This song... The playlist has gone full circle.
Literal
Huh? This song... Playlist [subj-が] one-loop doing [progressive-てるな].
プレイリスト 'playlist' is a loanword. 一周する 'to make one complete loop/circuit' (一 'one' + 周 'loop'). してるな is the contracted ~しているな with the masculine musing particle な. あれ？ signals sudden mild surprise. A natural moment of noticing — the music has looped back.