Here's a way to save time and energy when repairing floorboards. Instead of drilling and chiseling damaged floorboards, use a plunge router. First, use a magnetic nail finder to be sure there are no nails in the way. Set scribe lines across the damaged pieces. Then, run the router across the boards for a quick, clean cut.
There’s more to soundproofing than just insulation. “Blocking” noise with a non-hardening caulk to close off airborne sound paths. “Breaking” the sound path using resilient wood studs with metal clips. Last, isolating and eliminating “vibration” with special acoustical matting that seperates the wall structure from floor vibrations. You can get all these items in soundproofing kits for both interior and exterior residential walls. Besides blocking most noise transmission, one obvious benefit is better energy efficiency, with greater “r” values.