From little I know, here are some suggestions. I assume you are using the PC, not a Mac. If you are using a Mac, specially the older ones, you need a pre amp with the mic. If you are using PC or newer Mac, make sure you have the mic connected to the Mic input, not line input. Although either could work theoretically, I have seen some differences in input impedance that could make the input sound terrible or low.
Secondly, as others have suggested, using music software (can't make a recommendation here sorry on PC side) and see the wave of the vocal part. See if it has high signal or not. That should tell you if the voice has been recorded properly. If it looks good, it could be your output. Make sure your headphone is plugged in to headphone not audio out. Again, this may or may not make a difference.
If you find it's the mic in general, my suggestion is to bypass the analog and go digital. I have used Sansom USB mic and it works great and provides a pretty decent sound input.
Here is one on sale at Ebay for about $70. That's one way to bypass sometimes unreliable analog mic input.
Just a suggestion.