I purchased my used Volvo S40 and after a week of rain, I noticed there was water swishing in my car. I could hear it, but no evidence of water in the front of the cabin on the driver and passenger side. I just happened to grab something under the seat under the rear passenger side and noticed my floor mats were soaked. I checked YouTube and saw this is a common problem so I had my cousin (whose a mechanic) look up the solution.

We went to our neighborhood home improvement store and bought drain tubes. He cut the car's drain tubes and used the one we purchased to help extend them. The issue was the drain tubes kept coming out, as the tubes aren't long enough. He also had to take out my seats and the carpet to HVAC the water from the cabin. Not sure how much this will cost if you have the dealer do it, but if you can find someone who works on cars and can take out your seats to let the car air out, you should be good.