There have been a lot of questions about this one -- I regret making the decision in the first place, but after like 43 or so, the default soup playlists were removed (with the intent being that you could choose which ones you wanted). I shouldn't have done that because it's just created confusion.

So the deal is that the features is still there (a lot fancier, actually) but there just aren't any by default. If you want your By Artist, By Album, By Genre, etc back, check out

http://jempeg.sourceforge.net/faq.html

At the bottom is a "How do I make custom soup playlists?" question with some examples. I'll bring the defaults back in 46.

Mike