I guess you have checked the volume settings of your phone - so lets go to step two ;-)
since the phone isn't new, the earpiece speaker could be defective, the are magnetic and every tiny metal piece sticks on the speaker, and thats why the speakers volume is getting lower.
you can order the speaker on ebay, but you have to get the tools for opening the phone too - but i would first check another option.
do you have a nokia store or a nokia service point in your area ? - ask them what they want for the repair. it's a safer way to get it fixed.