How can Firefly Reavers be "smart" enough to drive spaceships? Why, then does their radio conversations sound like some EVP directly drawn from hell?
Reavers are psychotic, not stupid.
From the Firefly wiki: (potential spoilers if you haven't seen the movie yet)
Reavers originated from Miranda, the outermost planet in a distant solar system. The Alliance had been testing behavioral modification on this planet through the chemical G-23 Paxilon Hydrochlorate (a.k.a. "The Pax" (Latin for "Peace")), administered through the air recycling systems. The chemical was designed to weed out aggression and anger. The ultimate result was that 99.9% of the population stopped going to work, talking to each other, and eventually stopped moving, eating or breeding, and therefore died. However, the remaining 0.1% ("a tenth of one percent") of the population had the opposite reaction, with their aggression and primal instincts boosted beyond simple madness into murderous rage, sadism and savagery.
So they're crazy and filled with murderous rage pretty much all the time. That doesn't mean they're incapable of higher thought functions or doing things like flying space ships. It just means they're beyond reasoning with after they get you in a corner.
I don't believe the radio thing is explicitly addressed in canon, but the idea that they're using their own encryption scheme, as suggested in the comments, is entirely possible. Once again; psychotic, not stupid.
I've recently rewatched Firefly, and in Ep03 "Bushwacked" I had the impression that the Reaver had moments of clarity in between his attacks of murderous rage.
Maybe not a canon explanation, but when Reavers are around their own folks they'd possibly be calmer, as Reynolds explains. Then higher thought can still set in, until they get 'hungry'.