Why does my cellphohone earpiece make things worse?

I have the cheapest cellphone, I think, that Motorola makes, the C139.
At least it is the one they sometimes give you for free when you start
a prepaid account.

When I plug in an earpiece, it doesn't work well. The sound is not as
loud and maybe not as clear. I thoought the sound would be both
louder and better with an earphone. What gives?

I'm going to test earpiece with friends' phones, and my phone with
friends' earpieces, if tney don't think my ears are too dirty. But I
could use guidance on what to look for.

Right now on my phone, I've used a one piece plastic thing that sticks
in one's ear, and I also bought a 15 dollar model at Radio Schack.
Isn't $15 enough?
http://www.radioshack.com/product/in...ductId=2991656
The one that hooks over the ear and has a rubber piece that goes into
the ear canal a bit. It's black rubber and grey plastic, and folds
for storage an inch from the end, at the mouthpiece.

"Discreet and comfortable.
The Body Glove Flex Headset allows you to enjoy extreme comfort in
extreme style. The discreet, lightweight design offers exceptional
sound quality, and the flexible ear hook hugs the right or left ear
for all day comfort. The unit folds up for quick storage and portable
ease, and operation is made simple with all the controls on one simple
pod." I object to the word "all". It only has one control, a button
that doesn't do anything with my phone.

Thanks.