As far as I'm aware I've never downloaded any plugin for this format to my winamp, which plays the mp3's just fine.
Then I checked, and the installed input plug-ins are CD/Line-in, DirectShow, MIDI, Module Decoder, MPEG Audio, MP4 demuxer, NSV, Vorbis, Waveform and WINAMP/WMA. The output ones are Disk Writer, DirectSound and waveOut.
I believe there's another, more likely, reason why this specific set of mp3s result in "there's something my directshow filters cannot handle" and would appreciate any suggestions! :-)