Dear reader,
here https://wiki.seeedstudio.com/Grove-Speaker/ in the schematic view, C6 is connected to input and ground, which will short-circuit higher frequencies (and draws current more than the allowed 40 mA of an atmega168).
Shouldn’t C6 be in series with input to remove DC component?
PS: If so, C6 could be smaller than 10 µF, because C7 is a 218 Hz high pass (with Speaker = 8 ohms {assumption}) too.