Makes sense. One thing I did notice, however, while using procdump, is that there is a lot of "Exception: 80000003.BREAKPOINT" messages popping up during playback. It spams a TON of these anytime an OGG or Opus file (at least most of them from my library do this, not all, I have found a few that just operate normally) gets opened by WACUP for playback (on play or track change). The address listed in the message never changes, it's always as shown for me. I also tested with multiple other popular audio formats, and it does not occur with them. This testing included AC3 stereo (which didn't work at all in Winamp), MP3, MP2, FLAC, WAV, WMA, and AAC.
Edit: I also noticed that WACUP will retain file handles on Opus and OGG media after changing playback to a different track, yet it won't retain handles on any other format after a track change. Maybe the Vorbis and Opus decoders are both bugged?
Also, the method I tried testing the AC3 files with was to use Nullsoft's DirectShow plugin to allow playback of AC3 content, but all it did was garble the output thinking it was a 5.1 surround sound track, when all it contained was stereo audio. Given the format was intended for surround sound, this was not out of expectations of what could happen.