Stage 1 try spraying a disinfectant like Lysol into the AC intake with the filter removed. There are actually AC coil sprays sold for this for residential applicaions. You'll probably need to repeat this at regular intervals. (I just went through Stage-1 last weekend, including a partial dashboard disassembly to try to temporarily exorcise the dirty sweatsocks smell from my '86 Porsche's HVAC. I last did it about 3 years ago, it finally became annoying enough to do it again. It took about 1/2 a can of Lysol, linen-scent because that's what they had at Lowes on Sunday. I've never found leather-scented Lysol, or even Castrol scented Lysol and I've looked!) The disinfectant should inhibit bacterial and mold growth until it washes off, which should be a function of how often the AC is used.

Stage 1 1/2 try to remember to run the fan with the AC OFF, with the fan ON for a couple of minutes before you park the car to dry off the coils.

Stage 2 the current generation of Volvos have a timer that runs the fan for a couple of minutes to dry off the evaporator coil. I've also heard of a similar device being retrofitted to earlier Volvos. Ask a good Volvo parts person. GM and Ford offer retrofit kits for their products. There are also mold-inhibiting evaporator coatings that many new cars have applied. It seems to be available on the aftermarket but you'd need to access the evaporator to apply it.

Stage 3 gasoline and a match.....

Good luck!

-Punxsutawney Phil