This kind of clicking sound to hear the flashing  with a loudspeaker I did already in the 60s in my Chevimp.

The capacitor leads the switching-transient to the speaker and make it audible. Then I didn't use a separate speaker, for the car has already a speaker (radio) that does the same and even better. It was directly connected to the flasher over a capacitor. But the sound was very pointy and not at all nice to listen.

My actual solution is shown in my thread, using a DC-operated beeper, connected over the flash contacts without a capacitor. Here the 12V drives the 12V beeper. Now it sounds

similar to a call.  - SERGE -

http://www.forwardlook.net/forums/forums/thread-view.asp?tid=23691&start=84

As I see the used capacitor is an electrolytic type 100uF/100V. The + should be connected on the battery side of the flasher . This value is not critical at all, about ok, could be somewhat smaller (10 - 20uF). A bigger value allows more lower frequencies to pass, but anyway inaudible with such a small speaker.

Using the car speaker, I had added a serial resistor of 22 Ohms, as far I remember, in order to prevent additional loading of the audio amplifier. The higher the value, the more the sound will lowered.

But the solution using a mini buzzer 12V (Shark Radio, about 5 – 10 USD) as described  is much more practicable and at all a typical alert.  – SERGE -