I've had my Blackberry Pearl since February. Never had any major problems with it. I don't drop my phone, or throw it, or anything like that.
The last time I talked on it was yesterday afternoon, and everything went fine. Today, I went to call someone and it was completely silent other than a glitch of the other persons voice every once in a while. So, then I called back, and put it on speakerphone, and it worked perfectly.
Not sure why the speakerphone would work and not the regular phone. However, you have a 1 year warranty. Forget figuring it out. swap it!
The speakerphone and internal speaker use different speakers. It sounds like there is a problem with the speaker making intermittent contact with the mainboard. This could be caused by either a loose physical connection, or possibly by corrosion of the contacts on the speaker/main circuit board.