Strange question...not so related to the Netduino forum.
However, here is my answer...
A 8pF capacitor is a drop in the ocean: simply you won't hear anything.
Since you should place the capacitor in *series* to the speaker, a *very rough* formula is given by the impedance (read as "resistance") of the capacitor at the desired cut-off frequency.
Supposing the speaker as a tweeter, that is a typical cut-off of 3kHz, you might consider the capacitor impedance as "greater enough" than the speaker itself. Might we say 5 times more?...Thus, 30 Ohms
Since the cut-off frequency of a C-R high-pass filter is:
f = 1 / (2*PI*R*C)
then:
C = 1 / (2*PI*R*f)
If you use the given values, you'll have about 8uF, which is one million times more than the 8pF one!
Anyway, that's a very rough way to calculate and filter a speaker. A bad calculation/adaption of the speaker impedance will stress the amplifier, and you could even damage it.
Good luck
Biggest fault of Netduino? It runs by electricity.