I have exactly the same problem, though I wouldn't touch Crescendo with a bargepole and generally use Netscape's native plugin for midis.

All the other sites I use which have midis continue to behave normally. It is only Mudcat which has started to behave oddly; and that in Netscape (4.7.9) only. Using IE, Winamp plays DT midis quite happily, though I can't access the Mudcat Midi Pages through IE. Opera plays DT midis using some native plugin or other so far as I can tell, but again cannot access the Midi Pages. Hmmm...