Don't know about the B747, but on the B757 not only can you turn the vol down to 0, you can deselect all three radios, and can also detune them - there are quite a few "gotcha's" to catch you out - hence when it's quite I always give a little click on the mic every 5 mins or so to check the speakers are still working, and if I've really not heard anything for a while, then request a radio check.
I managed to lose contact with London once due to me being inexperienced, and having a muppet moment - I now do everything I can to try not to repeat that heart stopping moment when I realised what I'd done - but it doesn't guarantee it wont happen again sadly!
DW.