What would cause our 2006 S-40 to have all 4 floor boards be wet?

I would check the wiper seals at the bottom of the window .... sounds like when it rains the water gets inside the door then brains inside the cab

A common problem is water not draining properly from the sunroof drain. If you remove the driver-side pillar cover between the driver's window and windshield there will be a tube which is the drain. They drain is clogged at the end of the pipe and not draining properly

I have a 2005 S40. If it has a sunroof, you need to check the drains and make sure they are working properly. Poorly engineered system - esp. where the hoses fit into the drains in the corners of the dash. I have had mine "fixed" multiple times - never successfully. Finally, I fixed it myself by spending $10 on 6" of rubber tubing and waterproof silicone. Spliced an extra length of the tubing onto the existing drain hoses and used the silicone on the splice and where the hoses fit into the dashboard drains. This is a very well known problem w/ Volvo... Just Google "volvo sunroof drain" and you'll see results where numerous ppl have posted problems and also class action lawsuit. Good luck.