speaker sound is in and out

As you didnt give a year or model I'm going to guess that something was pushed into the trunk and has hit the speaker wires for the back speakers . if its door speakers the wiring in the door hinge often gets worn from door usage. This also often kills your power windows and door locks . There is also the possibility of loose wiring at the radio itself . The only way to find it is the start at the radio and work your way out checking the wire. Pay attention to the grounds . A bad ground will stop the sound as well. Hope this helps