I would actually think the opposite. If the music is on all the time the fish will get used to it, the same way that they get used to loud filters and powerheads. They are more likely to get scared with occasional loud music. I would be concerned about constant loud music during the initial acclimation period as this is a very stressful time and loud music could increase it. Perhaps start with elevator music and gradually increase the volume??
However, I know I couldn't stand much heavy metal!!!