Rocchet
Rather than thicker insulation around bathrooms. Ask about soundscreen. It's insulation that is designed to reduce sound travelling through walls and floors.
Yep, if noise insulation is what you're after, good acoustic insulation has different qualities than thermal insulation (for one thing, acoustic insulation has a higher density than similarly rated thermal insulation, for better absorbtion of lower-frequency soundwaves). Bradford SoundScreen is the market leader, but there are some other brands out there as well.
If you're really concerned about sound, you can combine acoustic batts with acoustic plasterboard, like Boral SoundStop or Gyprock Soundcheck. We're looking to do that in a couple of key areas - and it hasn't added much to our initial quotes.