Noisy floorboards

This could be due to debris underneath the flooring if Orginal flooring or potentially not fixed down correctly- or if you are talking about finished floor boards could also mean the ply/hardboard/underlay may not be fixed either? Would need a carpenter to asess the situation and go from there!?

90% lose floor boards more than likely old flooring nails come lose done a few of these just removed squeaking boards checked underneath and screwed down problem solved

My guess would be nails bouncing out from the joists and grit/debris between floorboard and joist. As for the cracking I would imagine that is split floorboards nakig a splintering sound. Sounds like you need some floorboards replacing and the floor needs refixing with screws instead of nails but impossible to say without seeing the floor. You would need a joiner ideally but I imagine most general builders would be able to do it.

The noise that you hear when you step on a floorboard (or floorboards) is usually a result of loose boards. The bounciness and movement in these loose boards causes them to rub together or onto a fixing nail or floor joist which, in turn, creates an unpleasant and annoying noise referred to as creaking or squeaking.

Probably worth Punching a few screws/nails into the plinths via the top of the board. This should secure the board and overcome noisy foot traffic.