Whenever you have a crackle, or kind of a static sound, it's one of two things. Poor connection, or a short.
I've seen the wires on speakers themselves come together and short out right at the speaker. It'll still play some sound but it's usually really quiet and will sometimes crackle.
Check and recheck all of your connections, and make sure it's wired up correctly.
One more thing that it could be, if the radio has a volume knob/dial, is the potentiometer that's connected to the knob.