The affect of music on the brain has been looked at a lot. However, scientists still don’t know that much about how the brain responses to music. We do know that there are parts of the brain that respond to music (mostly the same parts that respond to speech). We also know that there are differences in the brains of musicians and non-musicians when listening to music.
Parts of music seem to be processed in different ways in the brain, whether we are talking about the pitch of the notes or the rhythm. We have to put the two together to get a melody.
I’m not sure of the affects of music on brain activity in general if you are just relaxing, but one of the other scientists may know.
I love music and I love science, but I’ve only very briefly looked at them together. For a while I studied music synaesthesia, which is where people can see colours when they hear music. It’s a really interesting (and cool in my opinion) condition, I am quite jealous of people who have it!
Generally if you have music syneasthesia you will also have perfect pitch – as different colours are associated with different pitches, so if you hear a note, you’ll see the colour and therefore know what pitch the note is. It’s really cool.
But that said, I don’t know how music affects brain activity, even in these synaesthetes.
Sorry I can’t answer your question, it’s really interesting though!