Indoor air
Musty first floor
When the main floor smells musty, the crawl space and exhaust paths are often the real source.
A musty smell on the main floor is often air moving up from a damp crawl or basement, not a problem that starts in the living room.
Check under the house for bare soil, standing water history, and wet insulation. Check dryer and bath fans for outdoor termination.
A simple humidity meter helps separate "feels damp" from measured high relative humidity. Compare the problem room to a dry closet baseline.
Clean the source path before buying fragrance or paint. Covering the smell does not dry the structure.
Educational only — not a contractor inspection or bid. When work is structural, electrical, or involves unknown vent chases, hire a qualified local trade.