Generally 3D printers print in ABS plastic, which is what most headphones are made of. I can't speak to the specific composition.
The margins of the best ABS printers right (Makerbot 2X) now are about 0.200 mm.
There is a printer, the Form 1, which prints at about 0.05 mm by using laser lithography - but the material it prints in is a resin similar to acrylic and may not be stable in sunlight.
The restrictions with 3D printing do not seem to be so much in terms of precision margins, but issues with inabilities in terms of printing objects with overhang etc.
It's quite obvious that once 3D scanners are up to a certain fidelity, people will simply start copying headphone parts and reprinting them / modifying them.
The revolution will be 3D printed.