If you have a CD Player, you need to do this periodically...

I have been using a bulk tape eraser on my CDs since the mid 90s.  It makes a difference.  I remember back then I had a buddy come over with some duplicate CDs to mine.  So we were able to listen to his untreated discs compared to my demagnetized discs and then demagnetize his and listen again.  Since then I demagnetize all of my new CDs.  The bulk tape eraser was about $20 at Radio Shack back then.  Turns out it works.

CDs have a thin layer of Aluminum sputtered onto the polycarbonate disc.  Aluminum is paramagnetic.  We also figured some of the CD label inks may have metal in them.  Perhaps even weak spinning magnetic fields jostle the laser head.  Who knows?  Demagnetizing CDs works so i just do it.

My brother worked at Sony making CDs then DVDs until they moved it out of the country.  He says CDs have an analog component to them.  Part of it is that the time domain is mechanical (The spinning disc) but the part I don't understand is why does a clean disc play louder than a dirty, smudged disc.

My little brother was a Manufacturing Engineer at Sony for 25+ years.  He is a Purdue graduate but the little snot has as many patents as I do.  (I worked in both Aerospace and Automotive in R&D and Design/development for my career).  He took me on a tour of their CD manufacturing line back in the early 90s.  Back then it cost less than a dollar to make a CD as I recall.  The discs are injection molded polycarbonate with the pits that define the digital information molded in place.  So that part is very similar to the manufacturing of vinyl records- just instead of pits the record has the grooves molded in place.  Next the discs are inserted into a special vacuum chamber and pure aluminum is sputtered onto the disc just a few atoms thick to make a perfect mirrored surface.  A UV cured clear coat lacquer is bonded over the mirrored surface to protect the mirror finish.  I remember him telling me at the time that CDs were not considered archival because the aluminum is not perfectly sealed and can eventually oxidize.  CDs are estimated to last 100 years, he said.  So its not a bad idea to rip them into FLAC files.  I have a gold CD and I imagine it is the same process just sputtered with gold as the source material instead of aluminum.  Hopefully it will last a long time since it was expensive.  Finally, the labels are silk screened onto the discs and they are packaged into their jewel boxes.  He said they produced 3 million discs per day back then.  CDs had a 2.5 second cycle time and DVDs were 3.5 seconds.  The master disc is made by laser etching a photo film on a glass plate, the residual material washed away and then nickel plated.  This plating with the pits (digital information) is about 0.25 mm thick.  The positive master is molded from the original master disc and then the working molds pressed from the positive master.  

To the contrary aluminum is a paramagnetic material.  Take a look at the early mechanical speedometers used in cars.  They used a spinning magnet inside an aluminum ring.  The eddy currents from the spinning magnet applies a torque on the aluminum ring which then pushes a spring loaded needle around the speedometer dial.  The torque produced by the aluminum ring is proportional to the speed of the spinning magnet.  
Whether it is eddy currents or static, the more error correction needed while reading the disc the more the music is affected.  

I activate the bulk tape eraser then approach the CD label from the side.  I move the eraser back and forth to cover the complete label and then move away from the CD sideways before letting the eraser turn off.  The eraser stays in the plane of the disc.