It sounds like you have either a faulty speaker or faulty connections, and I would think the latter. It is a simple job to clean the contacts, or even replace the entire thing. This link below is for the replacement speaker.
http://www.sw-box.com/blackberry-pearl-8100-8110-8120-8130-earpiece-speaker.html

http://docs.blackberry.com/en/smartphone_users/deliverables/1217/userguide_bb8100_gprs.pdf
Below is how to dissemble your unit, above is your user and troubleshooting manual. http://www.ehow.com/how_5991365_disassemble-blackberry-pearl-8110.html
Also first try sucking the speaker out with a vacuum, sometimes they get clogged up.
Below is a video how to dissemble.
http://crackberry.com/how-disassemble-blackberry-bold

Hello. This has been a common complaint with this blackberry. There is no fix that you can do at home. Take it into the phone store and have the technician take a look at it. J.