Media Player 9 Modifies Music Tracks

MP9 seems to modify certain files when they are only played. These are mp3
files, stored in a folder for future burning to a CD. (I time stamp the
create and modified date/time for files where all editing is finished to help
me keep track of completed files.) These files are not in a playlist.

When I play this folder by clicking "Play with Media Player", and look at
the files in the folder using Windows Explorer, for one or more files, the
modified date/time is changed to the current date/time. I am not doing
anything else except playing.

What is Media Player modifying? Everything seems the same, including the
properties. It doesn't seem to create a problem, but I sure am curious.

Thanks for your help.