Well, I guess it was kind of embarassing, that's why I didn't want to tell. I wanted to actually change the thread to a more general "recommend the best way to start classical music" type thread, but unfortunately, it was impossible to change the thread title.
Additionally, I thought there would be no more interest in finding out what the song is.
It turned out to be the Verge's "Bittersweet Symphony." I knew I had heard it somewhere, but I didn't realize it was actually a rock song and not a piece of classical music. I only heard the beginning part as I was walking by. I stayed around for about two songs prior, and they were all playing rock songs (with cellos instead of guitars, I may add) , so I should have figured this one would have been a rock song as well.