Hi Niroshan,
Most audio codecs employ sigma delta modulation in their DACs. To determine if the noise observed on your plot is normal or not, you would need to measure with a spectrum analyzer (see http://www.ti.com/lit/pdf/slaa313), or implement a sharp low pass filter at 20kHz. In other words, performance (noise and distortion) shown in the datasheet is bounded within a speciifed bandwidth.
Regards,
J-