I am planning to install floating engineered wood floor on a concrete slab and I just bought one of the test kits from Lowes to test the moisture content and
pH of the slab. The pH tests at 13 which indicates high for alkalinity. How
much of a factor is that to a floating floor?

Thanks

With a floater, it doesn't matter. You can completely "isolate" the flooring from the slab if you lay down some 6 to 8 mil polyethylene and then the foam underlayment.

If I use floor muffler underlayment is that enough, or should I use
polyethylene in addition to that.

Thanks

You must use both. The floor muffler is NOT a vapor barrier First the plastic, then the pad, then the floating floor over that. Lap the plastic up the walls 4". Then, when the install is done, you can install the baseboards and trim the plastic from the tops of the base. Savvy?