There is a drain from the evaporator box that can become plugged over time. Under high humidity conditions the evap box can accumulate quite a bit of water when the AC is running. If that water is not draining out or is draining poorly, it can mix with dust and other detritus to support the growth of some pretty funky things that come with some pretty funky odours.
On a day when you have been running the AC for a while and the humidity is fairly high, park the car and let it sit for a while. You should get a puddle of water forming under the car as the condensate drains out of the evap box. On a hot day, my 2000 leaves a small lake and Saskatchewan is not noted for its humidity.
If you search, there are some fairly old threads describing cleaning out the drain hose from the outside. If its seriously plugged that may not be adequate. There are some equally old threads describing pulling the blower housing apart to get a more thorough clean. You may need to do that anyway if you have a serious mould farm going on in the evap box.
Air conditioning maintenance guys have sprays that they use to deal with the growth of undersireable stuff and there are also cleaners specifically for evaps. You might try an auto air conditioning place to see if they have those chemicals.
When I bought my NSX, it had a lingering 'aroma' in it. A few treatments with Fabreze did a remarkably good job of eliminating the residual smell after I cleaned the car up; but, you need to deal with the source. If the Evap is not draining, its going to be collecting water and acting like a petri dish for undesirable stuff.