Hi arannugent,
Welcome to the HP Forums! My initial thought was that it was a hardware issue because you the sound changed as you twisted the connection in the jack. Try setting the default audio device to your headphones or your external speakers and let me know the result. Below is a link to a YouTube that will show you how to do that.
http://www.youtube.com/watch?v=bC18hbPpFVQ
Thank you,