In reply to Tomtom:
> Consider first, that you should always be paying attention to your leader, with an appropriate amount of slack, ready to secure an unexpected fall at any moment.
While I utterly agree with you, I'd like to point out that you've used the magic word 'should'. Whenever people 'should' do something, you'll tend to find a sizeable proportion doing something quite different.
And so it is on climbing walls. The amount of shit belaying beggars belief.
If you can't hear your climbing partner (often the case on trad crags, especially on sea-cliffs) the danger level has just gone up, dramatically.
Nowadays, many people seem to regard sport crags as outdoor walls. They take their wall behaviour outside. And not being bothered about crag communication because of a controllable factor (such as music) isn't a behaviour that I'd wish to bring outside.
Lest you think I'm being theoretical/pedantic/nitpicking, consider this:
Recently someone told me a horrifying tale involving 'crap communication, but it doesn't really matter'. The guy on top (of a trad route, with belay slings round trees) thought he was going to get lowered ('cos I get lowered at the wall'). Those on the ground thought he was going to take the rope in, so they could come up ('cos it's a trad route). He stepped off and went the length of the route. As the (unbelayed) rope whipped out, it caught on a tree root and saved his life. He thought it was his mates pissing around, having a laff ('cos we do, indoors').
Mick