How can I deduce the power of a capacitor from its datasheet?
Capacitors don't have power ratings because, ideally, they don't dissipate any power. They store energy unlike resistors which consume energy, giving it off as heat.

Instead, you need to consider the following:


The voltage rating needs to be at least that of the maximum voltage they will see in service.
For power regulation and loudspeaker connection electrolytics are suitable. Observe polarity.
The impedance of the loudspeaker decoupling capacitor needs to be low in relation to the speaker impedance. You can calculate the impedance at any frequency from the formula \$ Z = \frac {1}{2 \pi f C} \$ where Z is the impedance (ohms), f the frequency (hertz) and C the capacitor value (farads). Choose this for a reasonable bass frequency cut-off point. (Remember that frequencies below this will fall off gradually rather than a sharp cut-off.)