Pin 5 of a 555 actually needs a VOLTAGE to change the mark-space output of the chip. The capacitor on pin5 is just to hold the pin at a voltage that is 66% of rail voltage as the chip has three 5k resistors in series and the join of the top resistor and the middle resistor is connected to this pin.
It "holds" the voltage on pin 5 and does not have any effect on this voltage.
But it does prevent spikes entering the chip and causing timing problems.
You will not achieve anything by modulating a low value capacitance on this pin and it will need a high value cap to produce any change in voltage.
The BC 108 transistor is connected as an emitter-follower. This is the same as saying COMMON COLLECTOR STAGE. Basically the emitter follows the base exactly, but the voltage on the emitter is about 0.65v lower than the base. The purpose of this type of stage is this: You can move the base up and down "with your little finger" and the emitter will move up and down with about 100 times more strength.
Now the value of the electrolytic on the base:
If you move the left side of the 10u electrolytic up and down, very slowly, with your finger, the electrolytic will start to charge during the "up cycle" and not all of your movement will be transferred to the base. The same applies when moving down. To make sure the base moves up and down as much as possible, the value of the electrolytic is made as large as possible. The 100k is there to pull the base up a small amount so that you can move the base up and down via the electrolytic. The actual amount of "set" produced by the 100k depends on the gain of the transistor and the value of the emitter resistor. You simply change the value of the 100k until the transistor sits at about mid-rail.
Can't be described any simpler.