Thanks for the replies. I called the ecopink people (
http://www.spec-net.com.au/company/insol.htm) and spoke to a very helpful guy who gave what sounded like good advice for me (without rock guitarist teenagers, but wanting to improve sound insulation)
1. Sound insulation and thermal insulation require different products so don't expect one to do the other's job
2. Between floors, fill the cavity with their basic batts (about $12/sqm)
3. In wall cavity, use their basic batts (about $20/sqm inc labour)
4. Don't worry about getting special plasterboard
He had done this to his house and reckons the result is equivalent to brick walls.