oops, sorry, you said doors.
Modern hollow doors, and thin internal doors, are not much good at muffling sound.
I am very fond of panelled fire doors, which are a bit thicker, and much heavier. They form a much better barrier to sound, especially noticable on a bedroom or bathroom.
The soundproofing can be improved by injecting expanding foam (preferably the fireproof grade) between the door linings ("door frame") and the wall, where it will prevent the passage of noise, as well as draughts, dust or smoke through the gap; and fitting the "brush pile" edge seals which do the same. As fire doors are thicker and heavier, they need better hinges (lift-off hinges are very convenient) and often a new door lining. Door linings are very inexpensive, and are available in a variety of sizes. I would always recommend getting a local joiner in to fit them, he will do it faster and better than a DIY handyman. My old dad got a hernia from hoisting a heavy door around.
I bought this sort, made by Premdoor, as like the painted 6-panel style, but there are other suppliers. I don't like the fake woodrained ones.
It is much easier to paint a new door lining than an old one. You will need new architrave as well. Fire door linings have a groove for the furry strip.
The foam holds the lining firmly in the wall, and prevents it rattling or coming loose when slammed by teenage daughters. Noise comes through gaps.