In reply to crayefish:
> If no one is around... why not? But if someone turns up it would be polite to turn it off or ask if they are ok with it (assuming most people would be too shy/polite to ask).
This, although I think there is a judgement call as to whether you think it's fairly obvious that someone would rather you weren't playing music/wouldn't say if they did care, or someone who you think genuinely wouldn't mind, but you're just checking.
There've been a few occasions where we've had some music on either on lazy "rest days", or early morning epics where a bit of music gives atmosphere and psyche to the group (particularly when climbing in a 3), but pretty much always turn it off without asking if someone else turned up. This was usually either midweek climbing trips or really early starts in the middle of no-where where we were fairly sure no-one would hear it.