Audio from Arduino Due's analog outputs using LM386
Well, it's shown as a gain of 200 amplifier, and the signal can be 1.18V RMS, so if it was amplified by 200 and fed into an 8\$\Omega\$ speaker, you'd have almost 7kW going into that speaker. That's not what's going to happen, fortunately. 

The LM386 can't output more than Vs/2.8 RMS (+/- Vs/2 peak) volts so you don't need to worry too much. Once the output gets close to Vs or GND it stops. 

It will start distorting horribly if you overdrive it (turn the volume up too high) and make it hit the limits. That's called "clipping". A ~100K resistor in series with the input would keep that from happening, and keep the input within the +/-400mV range. 

You're correct about the purpose of the capacitor and about the input impedance.