Couldn’t you convert the AAC file to an MP3 file at home, put it on a jump drive, and then bring it to work? Or else, get a small speaker to plug your iPod into and use that at work? At the very least, I would think that would be safer. I wouldn’t trust those pirated music sites not to give my computer some sort of virus. :o
For me, I’m still not 100% convinced that dowloading music illegally is technically “stealing.” I believe it’s sinful for other reasons, but I’m not sure how the transmission of data from one person to another (without the original owner ever losing possession of the original) is stealing in the most technical sense. Since the capacity for digital file sharing is a recent phenomenon, I don’t think that the moral theologians have really tackled it completely. I would like to see that done.
If we were talking about stealing, it would be sinful. We simply have to look at an analogous situation with tangible copies of the music to see that. For example, if you had songs A, B, and C on CD at home, but your work only had a cassette player and forbade you from bringing in your own CD player, it would not be okay to go down to the local thrift store and steal a cassette copy of songs A, B, and C so that you could listen to them at work. Thus, if downloading music illegally is stealing in the technical sense, it would be sinful to do what you have described.
If downloading music illegally is some other type of sin, then perhaps one could justify it. I’m not sure, though.
Of course, all this is my own pontificating based on my understanding of Catholic moral teaching. I could be wrong.