There are so many companies out there that do air duct cleaning- I'm sure they do the air return vent as well. I would definitely call one of them to come out and clean. I'm sorry that I don't have a specific recommendation as I've never used one of those companies (they usually do carpet cleaning too), but probably should. They should be able to take care of the mildew for you, I would think. Not good for allergies, mildew. It could mean that you have a leak, as well- so I might actually call the A/C people first, I dunno. I mean, it's not normal to have mildew in your vents, unless there's a water leak getting in somewhere. The reason you don't smell it when the A/C is on is that the A/C dehumidifies. There are companies out there that test for leaks in your system (but it's usually air leaks from the vents, not water leaks into the vents).