Is it all speakers or just one?
If it is a single door speaker turn the radio on, open the door, and wiggle the harness. If you get the speaker coming and going then replace or repair the door harness as needed.
If it is a single speaker not in the door, a door speaker not effected by the wiggle test, or all speakers gain access to the effected speaker, turn radio on, with no sound coming out of it check for AC voltage at the speaker plug. If you have anything above .3V AC replace the speaker, anything less replace the radio. If you get exactly .3V AC turn the volume up and test it again.