It should work from bash, which is apparently your default shell. Forget the ksh parts. I suspect, though, that the next problem you're going to run into is you don't have a MANPATH defined. You can check that with

echo $MANPATH

and if it displays nothing then your man path is determined by a configuration file. There's a way to make that work, too, but...

Someone below mentioned the command apropos, which I believe does the same thing, only simpler. Try this...

apropos vorbis

Apropos tends to spit out a lot of really nastily formatted stuff, so it may not be simpler afterall, but what it will likely display for apropos vorbis is "vorbis: nothing appropriate".

Amy