It could be quite a few things.
For starters are you sure the radio is on?
I've sat in cars wondering what the problem is until I realized that me, or the customer realized it wasn't turned on. Other than that:
*Check fuses. I don't know if S197 had audio and sperate radio fuses but I believe the previous generation did. Since it is the shaker system you may have multiple fuses just for the audio.
*Make sure the radio is giving out an audio signal. Is it an OEM replacement or is it aftermarket? How was it replaced? If they used a wrong wiring harness or just spliced in there could be a problem there (this is for aftermarket). If it's OEM replacement than to replace the radio is not that big of a problem and I'd say the problem is located elsewhere.
*Check the audio connection from the radio. If you have an extra speaker, wire it directly (splice into neatly) to the radio and see if it plays. If it doesn't you have another bad radio. If it does play, either something is wrong with the speakers, or more likely so the amps. -
*Using the same testing speaker splic into the out put audio of the amps. If there is sound from the radio but none after the amp your problem is the amp. If there is audio up until this point, than the speakers might be bad.
*Make sure the amps and speakers are properly connected, someone could have just unplugged them doing work and forgot to plug a harness back in.
There's a starting point.