We test the sound quality of the boards with the loopback test of Rightmark Audio Analyzer. Do note: this means we test the quality of both the input and output of the on-board sound cards simultaneously. The 'weakest link' determines the result.
The dynamic range and noise level test are very comparable and show nearly identical results. The dynamic range test measures the difference in volume between the loudest and softest observable signal, the noise level test measures the difference between the loudest sound and the noise level.
To put the numbers in perspective: the highest dynamic range c.q. noise level theoretically possible with 16 bit sound is 96 dB. 99.9% of the sound you listen to on your PC (CDs, MP3s, YouTube, games, ...) is 16 bit and the graphs proof that the hardware is no bottleneck in almost all cases. We can only achieve the higher scores by testing with 24 bit audio. By the way, we cannot imagine someone actually able to distinguish between 91 dB and 100 dB signal/noise ratio.
In terms of sound quality ASUS is the best: the ROG motherboards feature an excellent dynamic range. The MSI XPower and the high-end ASRock boards also score quite well. The cheaper boards with older Realtek codecs can clearly be distinguished by their shorter bars.
The stereo crosstalk shows us the extent to which sound for one channel (left or right) can be measured on the other channel.
Last but not least, we measure the total harmonic distortion, an average of the distortion throughout the full harmonic spectrum.