Thanks for all the advice - I wonder whether foam underlay for carpets might do the job? I can probably lay my hands on some of that quite easily.
I was going to do this in my daughter's bedroom as its the one without a direct wall onto next door - but it looks as if it'll have to be the cats' room as its smaller and inhabited only be felines + gerbil (safely tanked, but hungrily coveted).
Actually - I might be panicking unnecessarily - I think I'll do some experiments with the minidisc tonight to see how far the sound actually carries. I'll experiment with hanging some duvets around the room.
I like the idea of those egg-boxes though. If they were painted bright colours in different planes they could be really psychedelic wallcoverings. Skipjack, you may have invented a thing of genius!! And then if you put LED lights into the hollows & wired them up to light up randomly - or in time to music....

Cheers

Kris