99 nissan altima radio on no sound

I installed a new radio in my 99 altima it was installed correctly and worked perfectly for about 4 months then one day the radio was on and working properly (cd player, sat radio, radio, ect.) but no sound came out. I removed the radio and checked all the connections and they were all good so I assumed that it was the head unit. I put the factory radio back in the car and it worked and sound came out but after a sort time (less than one day) the same thing happened; the radio worked and functioned properly but no sound came out. I checked for factory installed amps and couldn't find any. I checked all radio related fuses and they were all intact. Please help if you have any suggestions I'm sick of listening to the sounds of my heater.

Did you change any of the speakers in the car?

sounds like you have a blown speaker somewhere.

yes, I changed the front speakers

 you may have a shorted speaker wire..........I would unhook one speaker at a time and see if the sound comes back on........ I would start with the front speakers since those are the ones you changed