Audio Noise: ATMEGA328P, MCP4921, and LM386
Saturation.

You connected the reference voltage of MCP4921 to its \$V_{DD}\$, through a filter. This means that \$V_{REF}\$ is either 3.3V or 5V (I guess 5 V, because of your LTC1514-5).

This means that your analog signal has a maximum 5V peak to peak value. The LM386 in that configuration has a gain between 20 and 200. Assuming a gain of only 20 (which I doubt, as there is a \$10\ \mu F\$ cap), you get \$100\ V_{pp}\$, which of course can't be achieved, also because the LM386 maximum operating voltage is 12V.

You can try:


inserting a resistor in parallel to DEC_C6, to reduce the reference  voltage (it can be as low as 0.04V). Still noise performance will drop I guess.
Attenuate Vout_A. But again, not only the noise performance will drop, but it would be a very inefficient solution: you attenuate the signal and the you amplify it back.
or use some non amplifying speaker driver configuration, so that it
won't saturate.


Anyway keep the 386 away from the switching regulator. In fact the 386 is very non linear outside its bandwidth, and therefore the 650 kHz noise could be picked up and brought in band.