Found a long discussion here that basically says it's a bug and many have had success in getting t-mobile to replace the phones.
Cant hear audio when placing / receiving calls - works on speakerphone though - RIM Blackberry Forums - Pinstack.com
BBrepairshop.com - BlackBerry Repair Parts for BlackBerry 8100 Pearl says they might can get a replacement speaker for $25 but no guarantees that'll fix it. They offer free estimates if you send the phone in.