I think some people are getting alot more uptight than this question called for.
Whilst I don’t consider myself an 'old hat' in this business I have been a professional pilot for some time now.
I fly for a company specializing in airborne survey operations. We have a very nice system integrated into the aircraft’s intercom. We plug in an MP3 player into the system, which plays music into all of the connected headsets but cuts out if any of the crew talk, or a radio transmission is received.
Obviously the system is switched off during critical phases of flight but in my experience having the music is VERY MUCH appreciated during the long hours of surveying a large area. I don’t think it affects my, or any of my crews concentration in fact I think it actually helps us stay alert during the longer sorties.
I honestly don’t see how a system like this, if used properly, can possibly affect safety in a negative way?