As other posters mentioned, two of the opamps create a virtual ground. (Op amps generally have optimal performance when the input is halfway between the power

As other posters mentioned, two of the opamps create a virtual ground. (Op amps generally have optimal performance when the input is halfway between the power rails, and the two batteries in series don’t guarantee that will happen). The other two each drive one channel. The 0.1 gain is to reduce noise created by the DAC, which is independent (in this case) of the volume control. So the author is turning up the volume on his device, to compensate for the 0.1x gain. Accordingly, DAC noise at the speakers is 1/10th what is was before, but the signal is the same.