There was a thread on this a while back, with some folks claiming that if you post to a thread without reading every word already posted to that thread, it's self indulgence, whereas if you read first so that your reply answers questions raised but not those already answered, it's communication. Seemed a bit oversimplified to me but it made sense...
About musicians, since I'm not one but rather an audience, I find that when a band/DJ whatever keeps their metaphorical finger on the pulse of the audience, it's communication, whereas those that ignore audience reaction and don't recalibrate their offerings to them, are being self-indulgent. Case in point: The Blues Brothers at that redneck cafe, tried their music, didn't work, switched to Rawhide - now that is not self-indulgence (though the beer, maybe, was!).
Have you ever seen a dance floor clear in under 6 seconds when a DJ/band started something particular? The guy who fades it out fast and puts something more like what was being enjoyed, even if they LOVE LOVE LOVE the song that's clearing out their audience, is communicating.
All this, of course, prefaced with the usual IMHO disclaimer.