Summary: There was always a constant humming noise in the output of speakers of both the audio channels. Tried to resolve the problem. Details are given below:
Details: The source of the noise was the typical 60 Hz (and harmonics), ~13 mV peak to peak output, in at least three channels of the DAC. (two coming from the DAF module, and one not related to the DAF.) Attachment 1 shows the noise in both the DAF channels. As compared to that, the signal coming through the AGC weak, about 6 mV RMS, about the same order as noise. In order to resolve this, the gain of the AGC was increased, so that the RMS output voltage of the Fibox (FBAO, the one at the output) was about 1.23 V RMS. It is approximately equal to +4 dBu, which is the typical expected output of the Fibox, according to the datasheet.