Surprisingly, I don't pay much thought to it. Sometimes I'll have one song on loop forever. Or even the sounds of a digital radio modem in the background hissing, chirping and clicking away. Other times, I just turn on some relatively low-brow commercial FM radio station because I can't be fussed to choose music. It serves the purpose of keeping the room from getting too quiet.
I have a list that dates from the early 1940s through the early 1970s. It is shuffled and random so it can go from Glen Miller to Deep Purple in a heart beat. I however get so focused sometimes that I come into the Lab and turn on the stereo, cue up the computer and then don't realize that I forgot to start the music until two hours later when it is time to go to lunch.
