According to PicoCOMA5_Hardware_eng.pdf:
The onboard soundcodec does support a stereo analog input and a stereo analog output for 1Vpp audio signals.
Anyway audio output volume is very low, it can be decreased even more by amixer 'PCM' but cannot be increased.
Output voltage on audio output is about +- 0,1V.
Is there another setting for audio output volume?