Michael Sayre |
I'll play devil's advocate here; the rules don't actually cover things like decapitation or amputation, so those are largely cosmetic changes with no more mechanical impact than the GM chooses to give them (unless you're using an alternate subsystem like the one Skulls and Shackles). Meanwhile, a familiar's ability to speak with its master or creatures of its type is both magical in nature and clearly defined by the rules. So, if a familiar who has somehow lost its head is still capable of taking actions and serving as a familiar, then it's still capable of speaking to everyone defined by its abilities.
***Edit***
Just saw Kalindlara's post. If it's a specific creature type called "beheaded" I'd start there to see if the rules for a beheaded define its capacity for speech.