I see it all the time on the SRT, and I think it's a bad idea. I had an incident last year where I called "on your left" to another cyclist I was overtaking only to have him almost hit me when he went left around a puddle on the trail. That was the point I realized he had earplugs in and it was clear he never heard my call.
If you must listen to tunes when riding, at least keep your left ear clear.