At a "high Level" AGND and DGND are the same thing, if you have a system that has both types of ground, and you use an ohm-meter to measure the DC resistance between the two grounds you will find it is very close to zero ohms. The two types of ground are tied together at one point somewhere in the system. The point where they are tied together is called the "reference ground point". Since the ground wires and tracks are not truly zero ohms resistance any ground currents flowing through the tracks/wires will create small voltages. These small voltages will get amplified by your amplifier and come out as noise. Ideally you want to connect your AGND directly to the "reference ground point", this way there are no stray currents flowing through the wire from your amplifier to the "reference ground point". The DGND on your amplifier is used for the power supply connections and for digital signals and is not sensitive to the very small voltages caused by currents flowing through the ground circuit. DGND can be connected to any convenient ground point.
Now to your specific question, where to connect the DGND and the AGND on the DragonBoard. I am going to assume you are using an analog amplifier (because it has an AGND). The DGND signal can be connected to the GND signals on the low-speed connector, or even better we have provided a convenient ground on J7 at pin 4 (see Schematic page 30/34 here: http://linaro.co/db410c-schematics). I am also assuming you want to use the headphone signals to drive the inputs to your amplifier, the right and left audio signals are L7 pins 8 and 10 respectively (CDCHPH_R and CDC_HPH_L). Finally you need to create the "reference ground point" by joining three signals together at one point. Connect AGND, DGND and J7 at pin 9 (CDC_HPH_REF), the point where these three signals are joined together will become the "reference ground point".
For best possible audio performance, you should also connect the reference ground point to the shields on the co-ax cables that transmit the analog signals to the amplifier. If possible, do not connect the cable shields at the amplifier, since they will form a ground loop. Anywhere there is a loop, it acts like an antenna and can pick up electrical radiation which will be amplified by your amplifier.
If you can tell me exactly which amplifier you are using, I will try to mock up the connections and do a sketch of how they should be connected.
Full Disclosure, I am an employee of Qualcomm Canada, any opinions expressed in this or any other post may not reflect the opinions of my employer.