"what kind of applications require real time these days?"
If Gleixner, when he quoted those download statistics ("About 30% of those went to European corporations, 25% to American corporations, 20% to Asian corporations, 5% to academic institutions"...), had also disclosed who those corporations were, this probably would give a good real-world answer to your question - or at least to the question: "What kinds of applications is the Linux realtime kernel being used or considered for these days?". Has he released this info? I suspect not - I suppose he's justifiably paranoid about being sued for breach of privacy or something like that.
" Linux audio used to be mentioned often, but these days it appears that CONFIG_PREEMPT is enough; is it?"
I can't answer that, but I will speculate that since music and MIDI recording and playback requires hard realtime capability, any companies considering using Linux for this would tend to favor the realtime kernel over the mainline kernel, since although the mainline kernel, perhaps, does fine x% of the time, the realtime kernel is likely (or certain, perhaps) to do fine y% of the time, where y% is significantly greater than x%.