Hi Edwin, thanks for the feedback. I've tried to answer all your questions comments below.
"In the schematic I see failrly low values of capacitors on the outputs of the voltage regulators, specially the LDO's usually require larger values on the output side."
- the values are the ones recommended in the datasheets for those particular devices. The 9V LDO is only supplying the opamp so ripple is minimal so the datasheet spec'd value is used. The 3.3V LDO is supplying the codec also uses parts spec'd in the datasheet, but also note the codec already has 10u bulk tantalums on all the rails.
"The 3v3 for the audio chip gets its input voltage form a 5V regulator which is supplying the Teensy, why not put the input on the 9V?"
With LDOs, the heat produced is proportional to the difference between the input voltage and output voltage, and the load current. So you always want to feed LDOs with the lowest input voltage possible for normal operation to ensure the regulator doesn't need heat-sinking.
"I also like to use a larger cap over the 3v3 of the Teensy. I had the feeling the DAC output of the teensy sounded a lot better when I used an additional 470uF on the 3v3 output of the teensy. Maybe that can help you too."
If you are using the Teensy onboard DAC, having good supply filtering would be essential and the large cap might help if the Teensy is pulling a lot of current dynamically. . However, on the TGA PRO, the Teensy is digital only. All analog circuitry is handled by the CODEC which has dedicated/isolated anlog supplies. Any ripple on the Teensy 3.3V will not have any impact on ripple on the analog supplies they they are not derived from it, they derive from the 5V USB.
"In an application with a sensitive microphone we have a resistor in every dataline we used. The display update noise was quite bad, after using an 150 ohm resistor in every dataline of the display and audio codec it was way much better."
Note on the TGA Pro schematic all digital lines between the Teensy and the Codec have 33 ohm source-series resistors to reduce EMI / reflections from fast edges.