Mildew follows moisture and a lack of ventilation. Do a water spray down test to determine if there is any water seeping and staying down inside the doors.
Do you have a cabin filter ?
More details will come in handy.
Assure that your evaporator housing is draining properly.

Make sure that the drains in the bottom of the door aren't blocked; you might have water in there.

The weep holes at the bottom of the door are clear.

Earthworm, I'm not sure what a spray down test is, can you elaborate. As far as a cabin filter, do you mean an air filter for the car's ventilation system? If you do, the answer is yes. The evaporator housing is indeed draining properly (although this smell definitely comes from the windows).

If I use diluted non-chlorine beach in a spray bottle and spray the rubber weatherstripping around the window would that get rid of the smell?

I dont think I would put bleach around the weatherstripping it would probably deteriorate pretty quickly. I have this smell occasionally in my cars and find that leaving the windows down in the sun for a few hours generally gets rid of the smell.

We used to do this "spray down water test" at VW to trouble shoot leaks.
And also at BMY for the same purpose.
Set the water hose for a gentle rain-ish spray, attack all the openings(including the lights - tail-lites are know to leak).
Good to have a helper inside, but at BMY we used blue chalk on the door seal.
Any leak would then easily appear.
The VW vent windows were a prime leaker, little else was.
Many BMY doors were distorted, and the seal was too minimal, very old technology was used..