Not everyone sounds good doing recordings. Some don't know how to adlib (but think they do), some (rappers) think they can be like jay-z and just hit a beat without any practice, some singers are good live, but bad in a studio, and there is no mic in the world that can fix all these problems, and no engineer can tweak a knob and correct wackness. I bought a cheapo mic Octava 319 for $49 and it sounds GREAT when the right person is behind it. As long as your getting a clean signal into the recorder, there isn't much more you can really do. Sure you can compress, and add effects, but effects on bad vocals ain't gonna sound good.
I'm no recording guru.. in fact, I JUST started doing vocals at the end of December. One thing I have learned is that you want the expression to come thru the music.. I mean, I can't say I hate you, and have a smile on my face, with music, if it's a love track (for example) you want to be able to believe that she (or he.. heck, I dunno your steelo.. just kidding) really means it, and isn't reading the lyrics from a book.
In a nutshell, get the levels up there, but not maxxed out, & try to record directly into the recorder. If they still sound like crap, it ain't your fault. Give em a copy of the track, and tell em to practice it for a week.
. ..roxstar. ..
http://www.soundclick.com/creamofbeats