Radio sound sometimes and not

When car starts, radio turns on and shows cd playing but there is no sound, sometimes there is sound and sometimes not. the radio itself turns on all the times, but sound not always,radio was replaced but its the same. so is not the radio, also the alert sound that tells you the lights are on or ignition key its in, that alert sound comes from the radio and sounds from the driver side speaker, if a different gm radio its installed, that specific alert tone will be different, I installed mine from an impala and the alert sound is not the same as the other radio, both radios were tested in other chevy car and bot radios works right away with no problem, so bad radio is not the problem. any ideas...?