Almost never do you want an HVAC return in a Basement.
If it stinks down there you might have mold, etc. you dont want that mixing with the air you distribute in the house.
Dehumidify and ventilation is key. Pick a dry day, and open a window, then stick a fan in the furthest window from the window that open (cross ventilation). If its cold out get heat going down there, the hotter and dryer the better! You might need a high BTU propane shop heater.
Once its under control you can try to cut a HVAC supply in the basment to positively pressurise the Basment.
Also a radon fan works too, because its drawing odor and vapor from under the slab.