Headset jack issue? Sound gets muffled depending on plug orientation

I'm having an issue with my 32GB iPhone 3GS where depending on how I insert the headset, the sound may be muffled, usually the vocals and not the bass/guitars/etc. The problem goes away if I turn the plug around but it is getting a tad bit annoying. I am quite sure it is not a headset issue as the volume problem persists on all of my other earbud headphones.

Should I proceed to get it fixed/exchanged as it is still under warranty? I've done a search on Google and TiPB but can't find any other similar occurrences other than one on Yahoo Answers (sorry, can't link it as I don't have enough post count yet. First time iPhone user here;) )

Thanks in advance for any advice given!

If you are under warranty, I would definitely take it in.