I missed this too - I think putting a little play arrow next to the "+" is in order.
Other than that, I love it.
Further comment (because I've continued to play with it, because I think it's awesome):
I've found a couple of tracks where the time (in green on the right as it plays) was given as "NaN:NaN". That's probably not a desired behaviour. (fwiw the track was "Thunderbirds Are Coming Out" by TISM)
In which case, very impressive. I'm particularly impressed that it even finds mashups, and the interface (apart from the difficulty I had with getting it to play) is very nice.
I hope you've thought through the legal issues. It could really use a FAQ with answers to questions like "So where the hell is all this music coming from?"