They had a flooring company assess the floors and we were quoted about $4100 to refinish the whole first floor hardwood floors.  

I put roughly 13k into it removing wall paper, wood paneling, replacing a drop ceiling in the kitchen with a dry wall ceiling, refinishing hardwoods, adding lighting, and updating some plumbing.$1100 in rent for a 60k property might sound great, but property taxes on the house are roughly $300/month, so its not as lucrative as it might sound, plus the 13k rehab out of pocket is more than my down payment so it cuts my ROI in half.

I want to sand and refinish existing floor but the concern I have is noise traveling between units.  

If flooring and walls can be refinished to make them seen new, then no need to replace them entirely.

Of course, I do all the installations, refinishing, and maintenance, I get almost all of my hardwood floors for free upon acquisition, and I have low standards for what these floors look like.

My personal advice would be to refinish them and put a very hefty plenty in place if they are scratched/ruined.

I've done a good amount of renovations: siding/soffits/fascia, gutters, replaced all concrete work, repaired and refinished porch, all baths and kitchens, refinished hardwood floors, full paint job, landscaping ,etc.I live in one unit and rent the other two, a 3br/1.5 ba for $1200 and a 1br/1ba for $700.

If the tub isn't damaged, you could have a company like Get-a-grip refinish it.

Hi @John Stevens, not sure how accurate this is for you, but real hardwood flooring should last a lifetime but should be refinished every ten yrs. 

It cost them thousands to refinish when they moved out.