I think it’s very difficult to outright say always muttar or always assur with these issues. It seems to depend not just on who you ask but what, why, how or even who’s music and from what medium to what medium etc. I know two people who asked r’ moshe sternbuch about this, one was told that the specific copying he wanted to do was muttar the other was told assur. The differences in situation were very slight yet apparently significant enough to make one alright and one not.
I have wondered for a long time though, if one did copy in a way that was deemed to be assur, how would one be mekayem ‘v’heishiv es hagezeila’? Simply deleting the music isn’t returning it back to its original state as that exists on a computer or cd or ipod somewhere.