It'll serve a couple of purposes, I'd reckon.
The first, you correctly say will be for sound deadening, I'd also imagine it will slow-down any road debris caught in the tread and flung off the tyre, which not only presents a hazard to others, but can damage the car itself.
Problem is, sometimes anything like that can create moisture traps, which will allow small problems to become big headaches if left unchecked.
Personally, I'd carefully remove enough of the liner to avoid damaging clips, but to allow a visual inspection of the metalwork to be performed. If all looks well - simply replace it. If not, try
or renault for clips (some will break), and hopefully it isn't at the welding stage.
If you do need to correct rust, once it's all been treated, I would give the arches a decent coat of heavy underseal, leave it for a couple of weeks, so it gets a chance to go off, then replace the liners.