I've had the same problem with mine. Apparently this has been a problem but was supposed to have been fixed months ago. Of course maybe yours had sat on the shelf for awhile at the store and you got an older one. I do take some blame for the problems mine has because I used to just wrap the earphone cord around it and stick it in my pocket. I discovered that if I pushed back on the headphone plug good sound would return. Then I just wedged a small piece of wood (a flat toothpick in this case) between the headphone jack and the case so it would keep a constant pressure on the jack in the direction I needed. I then trimmed off the excess toothpick. It's worked fairly well for a couple of months now but I still can't put any tension on the cord or the crappy sound returns. It'll get me by until I get a microphoto though.
I had my zen micro 5gb for about 3 months now, I am getting really bad sound quality (it will get really quiet and sound all distorted) I am positive it has something to do with the earphone jack, becauase it does this with any earphones or speaks I try. Do you know of any way to clean out the ear phone jack? or possibly some other solution?