Cannot understand the attenuation in signal from PC to microcontroller(NI myRIO)
Surprisingly, I am getting an attenuation of the signal corresponding to the volume reduction in PC(not linear!!). Only I can see the waveform in microcontroller close to amplitude 1, when I increased the volume of the system to 100%. Please explain this !!

Humans don't perceive sound intensity linearly, but rather logarithmically.  For a series of doublings in sound power is perceived as a linear volume series.  This has been well known for over a century, so the people that designed the volume control in your PC software know this too.  To give you what you will perceive as a linear volume control means actually varying the voltage exponentially.  This is why the voltage drops quickly from maximum as the volume is turned down only a little.