Audio Systems: speakers not working?, external amplifier, volume levels

Question
i installed a new head unit in my car and half way though (before i installed it in the dash, i tested it and it worked. so i install it in the dash and now it doesn't work. what doesnt work is it doesnt play the music... the speakers DO work because i can very clearly hear the beeping noise when youre changing the settings
the clock works fine. i can change channels, modes, volume levels but the music just doesnt play. not on radio or CD
any input is very welcome

Answer
Is there an external amplifier in the car that must be connected up?  If the speakers in the car are already connected to a hidden amplifier they will not get sound unless you have an audio output signal connected to the inputs on the external amplifier.  Maybe that is the problem.

Let me know and we can take it to the next step if that does not solve the problem.

If you come back, let me know the car model and year and the system that you replaced and the new unit; that way I will know what we are dealing with.

Thanks
C