It is suggested that music can provide a background sound that cows get used to and overcomes any unusual sounds that may frighten the cows. As we know, anything that frightens a cow or makes it uncomfortable can cause an increase in adrenalin which interferes with oxytocin release and reduces milk letdown. There are reports that cows respond better to certain types of music. However, it is probably more important that the music be consistent since cows respond best to a standard routine.
Dave Fischer, Extension Dairy Educator, University of Illinois - Illini DairyNET