555 and loudspeaker - Beginner question
Typically, the sound of a metronome is not a short tone burst, like a beep or chirp.  It is a click or tick or tock - sonically it is an impulse, not a discernible note with a frequency.  That is why the circuits you have found send a simple, short DC pulse to the speaker.
AND - The &quot;really happening&quot; drawing above is incorrect.  There will be a sound impulse at both the rising edges and the falling edges of the 555 output waveform.  The two impulses will sound slightly different, because the main part of the sound is different.  One is a compression wave (the speaker cone moves toward the ear) and the other is a rarefaction wave (the speaker cone moves away from the ear).  There are harmonics, speaker cone oscillations, and other contributors to the overall sound, but the main part is what the cone does when the signal edge comes along.
If you really want to send a tone burst to the speaker, post your schematic for us to review.  In general, it is better to enable/disable the second 555 by driving its Reset input rather than switch its power on and off.