Sure, I know that protocol and astro droids are common in the Star Wars "world", but wouldn't the voice/attributes/personality be recognizable on 3PO in particular?
Don't know about LFL, but here's my stab... I would guess that after 20ish years, they probably don't recognize the traits of 3PO's voice, and especially since he got the gold coverings. I'm not sure when Shmi was sold to Clegg and for how long 3PO was in the Lars' possession before Anakin came, so it may also depend on the length of time he was at the Lars' household as well. Plus, people probably thought of droids as more of possessions, for labor, than individuals like the heroes did.