Normally I would say this is probably a heater core leak but since it's a brand new car I'm not sure what It could be. Does it smell like antifreeze? Since it is new, you should take it into the dealership. It should be covered under even if you are the 15th owner. It's a new Volvo still in it's factory warranteed timeframe. If the carpets somehow got wet and the plastic mats were over the them, it could possibly have kept in the moisture. Most likely, since it is summer, I am guessing you have had your air conditioning on frequently? If so, that is most likely the reason. The cold air in your air ducts under and behind the dash can condensate especially in warm, humid conditions and when you sometimes run the A/C with the windows down. This will produce drips, even puddles of "musty" smelling water. Put a piece of plastic or serran wrap from your floor mat all the way up to where the dash begins. If this is the case, you should see water roll its way down to the floor mats. Hope this answers your question.