The zener diode is there to provide a -5V reference. It can be replaced with a -5V reguator. The reason it is needed is that sound cards are single-supply so the sound card chip output typically rests at 2.3V, and full on is 4V and full off is 0.6V.
So, with DC coupling you must set the offset pot (r3) to counter the 2.3V DC offset (so the output is 0V), then set the gain pot (r4) to reach +/-5V during hard clipping.
each tl072 has two op amps in it, you can add more for more channels of DAC up to 8 (for an 8 channel sound card)