Wearing earbuds that block sounds from your environment can be dangerous whether cycling, driving a vehicle, or even walking.
On our tandem, I installed a Delta Cycle AH1000 Airzound Bike Horn (115 db) and even that wasn't enough to get the attention of a cyclist riding down the middle of a fairly narrow bike path. This was in farm country, with hay fields on both sides, no road, silent except for crickets and birds. I guess he had his music volume up pretty high. Oh well, if he doesn't care about his safety or mine, I shouldn't be surprised if he doesn't care about his hearing, either!