Does one normally use 2.1 (and not 5.1 or 7.1) for listening to pure audio content (not movies)?

Does the addition of surround speakers erode the quality of audio sound delivered through, say a CD player? If so, how?

Hi.It is entirely up to you. There is no correct method when listening to audio .Not all CDs are recorded the same .Some times the ambiance is hidden when listening to two channel stereo and your surround speakers may recover it which will add depth to your music.The quality of your sound is determined by how well your electronics maintain or degrade the signal.If your Source equipment alters,changes or degrades the original signal from the software ,no other component in the chain can fix or improve it so when the signal reaches the preamp it is already degraded and may be degraded again by the power amp.

EDIT. Ever since the beginning of Stereophonic Recording the goal has been to playback music with the realism of a live event. We have come a long way since then but we still haven't got to the stage where we are fooled into thinking that we are listening to a live event because stereo sound is subject to fundermental distortions of spacial information that even a 6 year old child can't be fooled into thinking he/she is listening to live music. When you listen to a live event ,say a concert in a large auditorium.You hear the direct sound from the stage first and a couple of milli seconds later you hear the reflected sound bouncing off the walls, floor and ceiling which gives you a sense of depth to the sound depending on how long the reverberation lasts.