Well, it would depend on the background music, and also if it relied on you were able to choice what kind of background music it was or you had to be the observer to someone else's. For an example, I'm not sure if I'd want sad, swelling music in the background during a break-up, or worse, having some kind of revenge song like
this play in the background. On the other hand, if it were something only you could hear, and it matched your life well, that would be kind of cool. Because even though it seems like it would be weird - if it's something we got used to our whole life, we wouldn't notice it. It would be something that was...well...in the background.
That being said, if there were a device that could give you background music after you've gotten used to life without it? I think it's called an iPod...kidding, they're not
that smart yet. It would have to have some kind of artificial intelligence to match the setting and your mood and it would probably be weird to get used to. Though it does remind me of when I'm writing - if I'm writing a fanfic set in Twilight Princess, I'll go to that game's background music and find something that matches the mood. Again, not exactly the same, but the music does help match the mindset.