The sound card can apply noise-reduction and/or anti-echo effects without your consent before the sound gets to Audacity. These effects, if present, can be switched off in the sound card settings … Perfect tone Fades loss after 4 seconds - #2 by steve
http://forum.audacityteam.org/viewtopic.php?f=16&t=41179&start=0
Acoustic guitar before-after noise cancellation on sound card switched off.mp3