I have used the sound reducing panels (just a different brand) before and it does help but it will also be a considerable investment to make a real difference. 

We have solar panels on one of our rentals.

If you build out from scratch (which rarely makes financial sense because you have to abide by new building codes with fire sprinklers and solar panels), you may want to check with an attorney because I haven't been able to get a clear answer if you can do that with existing tenants because of yard space.

I added new flooring , Paint new electrical panel , changed all plumbing fixtures, and a new bathroom downstairs and put in a brand new kitchen.

If I included utilities, probably ~$800/mo or $3,200 gross.My main concern with it is that it has one furnace to heat the entire place, one electrical panel, one water meter, etc.

I actually met you at your plant this summer when I picked up a trailer load of panels for a project of mine. 

It's fairly common although I would go ahead and cover it with something else such as wall paper or those fake brick/wood panelling boards they sell at Menards

For example in California we are required to add solar panels. 

Remember once you open those walls everything has to come up to code, that will for sure mean all new electric and panel, alot of new plumbing and HVAC and can mean little more annoying things like framing not being sufficient. 

I am also a Certified Eco-Green Energy Star Preferred Agent specializing in Green Retro fits and new Smart Panel factory stick built homes.