I would have that checked.

But a dehumidifier on continuously will help with the damp smell. Either hooked up to a drain or go down and empty it daily or as needed.

Right. Do you have wet carpeting down there? Water leaks? Damp sheetrock?

Is there a bathroom? Does it have a proper trap? Does it get used or is the water in the bowl evaporated?

I don't know about the mold smell, but the sewer smell is probably from a dry trap. The floor traps go dry from non use. The water is the buffer between the sewer gas and the house. Just put water down them every once in a while. My husband takes a garden hose once a year and flushes them out. They also make plugs for the floor traps, but if your basement gets flooded there is no where for the water to go since they are plugged.

The way we resolve damp odors in enclosed environments is by using circulating fans and an ozone generator (we also use this to remove smoke smells. Don't bother with air fresheners or any other chemicals since they just mask the bad smell.

As others have advised, the first step is to get someone to isolate the cause of the excess moisture. The smell will go away on it's own 99% of the time once the source of moisture has been resolved.