There's a lot of slack on this issue at my store. Never two earbuds - that's a safety issue. But TMs will often have one earbud in, even on the sales floor. I think it's rude to guests, but TLs don't seem to care. In the back room any time and on the floor when the store is closed some TMs will have their phones stuck in their rear pockets at full blast. I think it's rude to other TMs who perhaps don't like the same music, but again, TLs don't seem to care.
I like music, but not usually what my co-workers like. And I like quiet, letting my mind wander.