Cannot understand the attenuation in signal as the audio bit depth of adc changes
From NI myRIO-1900 User Guide and Specifications:


  The audio inputs are left and right stereo line-level inputs with a
  ±2.5 V full-scale range
  
  • For Audio In/Out, 
  LSB Weight = 5 V ÷ 212 = 1.221 mV 
  Maximum Positive Reading = +2047 * 1.221 mV = 2.499 V 
  Maximum Negative Reading = -2048 * 1.221 mV = -2.500 V


Your computer's head phone output probably has another constant of proportionality between voltage and numerical value. Perhaps you can find manufacturer's data, but it is probably safer to simply measure with the help of your NI device.