Why such unusual RC filter?
A loudspeaker or many other inductive loads may be modelled as resistor + an inductor in series.   The impedance increases with frequency.  

Adding a properly dimensioned "Zoebel network" / "Bocherot cell" in parallel with the speaker etc tends to make the impedance constant (or is an attempt to). 




Wikipedia: Boucherot cell correcting loudspeaker impedance


The capacitor causes the resistor to have increasing loading effect with increasing frequency "compensating for the series LR.  

Properly designed, R = Rspeaker and C = L/R^2



Wikipedia
From here as Zoebel network OR
From here as Boucherot cell 

Wikipedia:


A Boucherot cell (or Zobel network) is an electronic filter, used in audio amplifiers to damp high frequency oscillations that might occur in the absence of loads at high frequencies. Named after Paul Boucherot a Boucherot cell typically consists of a resistor and capacitor in series, that is usually placed across a load, for stability.1

It is commonly seen in analog power amplifiers at the output of the driver stage, just before the output inductor. The speaker coil inductance of a loudspeaker generates a rising impedance which is worsened by the output inductor generally found in analog power amplifiers; the cell is used to limit this impedance.2  

The documentation for some power operation amplifiers suggests the use of a "Boucherot cell between outputs and ground or across the load".3

Additionally, Boucherot cells are sometimes used across the bass driver (and mid-range) of a speaker system, in order to maintain a more constant driving point impedance as "seen" by a passive crossover. In this specific arrangement, the Boucherot cell is sometimes also known as a Zobel network.2

Some loudspeaker crossover designs aim to stabilize impedance at high frequencies by including Zobel networks.[4]




Not just audio:

The diagram below is from page 5 in the ST data sheet referenced in ref 3 below.
It shows two x Boucherot cells used in a motor driver.
They say: 

APPLICATION SUGGESTION
In order to avoid possible instability occuring into final
stage the usual suggestions for the linear power
stages are useful, as for instance :


layout accuracy ;
A 100nF capacitor connected between supply
pins and ground ;
boucherot cell (0.1 to 0.2 µF + 1Ω series) between
outputs and ground or across the load.
With single supply operation, a resistor (1kΩ)
between the output and supply pin can be
necessary for stability.






(1)  Rane Professional Audio Reference Home - Boucherot
(2)  Rane Professional Audio Reference Home - Zobel

(3) ST, LOW DROP DUAL POWER OPERATION AMPLIFIERS, July 2003, Top of Page 5, right column. &lt;- The original ST link on WIkipedia is broken. 

(4) Bell Labs Journal: Zobel, O. J., "Theory and Design of Uniform and Composite Electric Wave Filters," Bell Sys. Tech. J., Vol. 2, pp. 1-46, Jan 1923.