Pozwole sobiez zacytować tekst z noty aplikacyjnej:
The interface also includes audio line-out signals, A_OUTR and A_OUTL, from the main unit. These signals correspond directly to the audio signals used for the speaker and headphone outputs of the main unit.
i tutaj:
If an expansion pack uses these signals it must amplify them for an expansion pack audio out function and connect the A_GND signal to the analog ground of the expansion pack
Jednym słowem sygnał jest za słaby aby używać go wprost, trzeba go wzmocnić.