Point it at the duke3d.grp file and set it loose. Might be a bit overkill for what you want, since it will extract everything. And you'll have to figure out after the fact what it is, since WinRipper will name everything DUKE3D_000.wav, DUKE3D_001.WAV, etc.
There's also Dragon Unpacker
. It can actually show you the filenames inside the grp file, though that may not be much help. Naturally, they're old 8.3 style filenames, and therefore pretty cryptic. Also, if you use Dragon Unpacker, you'll need something that can play/convert voc files (winamp works). WinRipper converts them to wav automatically.
Edit: Duh, there's also the kextract.exe command-line utility that came with the game, if you don't mind using command-line. Still have to have some way to convert voc files though.
You're always talking about Duke Nukem 3D though. Do you know if this works with Manhattan Project too?