Don't know about under floors, should be, but there are probably reasons why not.
However, I can confirm that it makes a massive difference when used inbetween stud walling blocking up an old doorway.
In my school there was a doorway into a classroom from a library. The doorway was no longer needed due to alterations, and finally we talked the teacher into losing the door!
Stud walling on both sides, a 9" gap between the two, packed out with loft insulation, a FULL bag of it, and double skinned plasterboard on both sides, skimmed off. The sound insulation is fantastic now!