As you haven't said what sound system you've got but you mentioned an amp. I'm assuming that you have a system with a head unit and a seperate amp in the boot.
So, if you've lost all sound it can't be one speaker connection so that's the speakers eliminated.
The head unit is fairly safe tucked up in the dash and you say that the cd is powering up and working correctly, and it's not easy to get to so . .
That leaves the amp that is sat in the boot and Land Rovers have never had a great reputation for being water tight.
So, by a process of logical elimination I would start with the amp. Get to it, get it out if you can and check the connections for corrosion etc.