I do not know why you talk about clipping (clipping occurs with an audio or video signal) when the output of your 555 is PWM rectangular waves that are already clipped by the switching output of the 555. Do you like to listen to the buzzing of PWM at a low frequency?
The BD136 PNP transistor at the output of the 555 will NEVER turn off because the output voltage of an ordinary 555 does not go to a voltage high enough. You should have used a voltage divider to drive the PNP transistor or an NPN transistor instead.
A "104" capacitor might be tiny but it has a lot of capacitance. It is 0.1uF which is 100nF which is 100,000pF. If the timing capacitor in your 555 circuit is 0.01uF ("103") or 0.001uF ("102") then the frequency will be high enough so it is not heard by anybody but the PWM dimmer will still work.
Do not feed amplified high frequencies into a tweeter. It is extremely fragile and will burn out.