Audio coupling capacitor removing DC
You are using electrolytic capacitors for your AC coupling.  Electrolytic capacitors don't perform well when they are reverse polarized.  

You have an AC signal imposed on a DC offset.  When you pass that through the capacitor, you lose the polarization - which side of the capacitor is "more positive" changes as the AC changes.  That results in distortion.

Try a 100nF non-polarized capacitor instead of your large value electrolytic capacitors.

Try that first.  See if it fixes your distortion.  

If the non-polarized capacitors helps but the cutoff is too high, then you can try using a large value ceramic capacitor - but they have their own downsides.

The capacitors and the load posed by the amplifier will of course create a high pass filter.  With 100nF and 20k, that will be 80Hz at most.  If the amplifier has any impedance at all then the cutoff will be lower.



What you are talking about is actually a "coupling capacitor."  

Decoupling capacitors are the ones you find on the power supply pins of ICs.