Electronic – LC Low Pass Output Filters for Inverters

Your output filter response should basically be the 20 dB/decade response of the capacitor.

The ESR zero will depend on the number and type of capacitors you're using. It's the natural corner frequency formed by the effective ESR and total capacitance:

\$ F_z = \dfrac{1}{2 \pi \times R_{esr} \times C_{out}} \$

The gain slope will swing more positive at the zero, and you'll see the phase changing as well. You can easily use a free SPICE simulator like LTSpice IV to model the output precisely (with ESRs, ESLs, etc.) and generate a Bode plot.

This does not make much sense. You said in comments that in PB4 there is a inductor and not a capacitor. Unless you did a incorrect assemble of your parts it should be a capacitor. The picture shows a capacitor and makes sense to be a capacitor. This is probably a low voltage and low power application. Usually you put low-pass filters into loudspeakers when you have woofers and subwoofers which, by the way, generally need high power. So it makes more sense to be a high-pass filter as Scott said, which is represented by a series capacitor. Depending on capacitor's value will filter more or less but still remove most of bass sound and DC levels.

As for the Vcc line, there is a capacitor (left) AND a battery (right). The 3.3v label should be linked to the battery. And this capacitor is now in parallel to the load. So it is a low pass filter and this is probably to help absorbing fast transients in Vcc line and keep it stable.