In case anyone wonders - I could solve the problem by applying the fixes mentioned here:
https://www.sparkfun.com/users/395251
insufficient delay after reset. In line 37, changing “delay(300);” to “delay(750);” worked for me;
if you rely on the playVoice() call: sendCommand() returns too soon, when the busy pin is not yet on. In line 94, tweaking “delay(20);” to “delay(50);” worked for me.