Other than checking the ring volume under your profile settings to check that it wasn't accidentally turned down, I don't have any suggestions. Maybe lint or something else is stuck inside the speaker???
My 8700 is actually probably the loudest ringing phone I have ever had.
In addition to my previous reply, how loud is the speaker on speaker phone? The 8700 has a reputation for being very loud in that department. It may help trouble shoot if it's a speaker issue...