Why is no ground plane used on the audio boards?


I'm designing a board that includes audio output, based on the schematic of the audio board with the SGTL5000 chips. I've noticed that none of the revisions of the board have ground planes, which is confusing - surely ground planes reduce electrical noise, and therefore produce a cleaner audio output? Should I also omit the ground plane from parts/all of my board, and why/why not?

The reason is basically because an unbroken ground plane is not feasible in 2 layer PCB design.

Opinions vary about audio grounds. But if you are using a 4 layer PCB, a solid ground plane is probably worthwhile. If using only 2 layers, a design which fills the many small unused spaces with ground probably isn't helpful and may even make matters worse. Maybe. Again, many people have differing opinions about how to design audio grounds....
Thanks for the reply. I guess it makes sense that a small board would have such problems. I am indeed using a 4 layer board so I will most likely use the inner layers as ground & 3v3, and leave the outer layers as just traces with no planes for the same reason.
Audio is very slow, a ground plane isn't so useful (RF and logic both benefit from a ground plane due to the importance of Maxwell's laws at those speeds) - star-grounding can be critical for audio though, as ground loops may pick up mains hum (very undesirable!).

Best practice with DACs and ADCs is to use separate ground planes for analog and digital, joined in one place under the chip (problem if you have several of them!). This is a crude version of star-grounding to keep digital supply currents from polluting the analog signals. And separate voltage regulators for analog and digital is important for the same reason.
Right, I see what you mean. In such a simple circuit with only a single component dealing with the audio signals, does it make sense to have a separate analog ground for the one chip and the headphone jack only? And on the subject of ground loops, would it make sense to use only one ground connection to the teensy then?
does it make sense to have a separate analog ground for the one chip and the headphone jack only?

SGTL5000 provides a "virtual ground" for the headphone jack. It's really just an amplified output of the DC voltage, so you don't need to use large capacitors to block the DC voltage of the outputs. This feature is explained in the SGTL5000 datasheet.

So to answer directly, yes, it does indeed make sense since you would otherwise need to add 2 large capacitors.
I found this helpful for input regarding ground planes and digital audio converters.

Regarding those capacitors Paul is talking about avoiding with the virtual ground - since the audio path is flowing through them they can cause noise or shape the sound. Ceramic caps can be used but at a cost of quality. Using larger sized smd's (1206) can help, but using electrolytic is better, film is considered best. With each option getting more expensive and having less longevity over time and temperature.

I found this article also helpful about the different choices for ac coupling (dc blocking) caps in audio.
Large value ceramic caps have a lot of distortion, more even than electrolytics, and are microphonic and never a good idea in an audio signal path. Film caps are used for non-coupling-cap applications as they have stable values, low distortion (especially polypropylene, teflon, polystyrene) - these are used in analog filters and tone controls for instance.

Coupling caps (aka DC-blocking) (should) have very little AC signal voltage across them so electrolytics can and are often used as the effect of distortion is minimal.