Kai, what does happen if due some reason (noise, power problem, program malfunction etc) a port pin goes to low level and keeps at this state for long time? For example, if MCU hangups while playing a sound? Maybe is it the reason to make scheme with some more transistors to make "middle dot" and capacitor-speaker-ground net from it?

Dear Oleg,

you are right, this IS a disadvantage of this simple circuit! But this circuitry is very very often used.
What would happen in such a case?
If current through loudspeaker is choosen rather small, nothing serious. But if bigger currents are choosen, this would be much worse, of course. Because of additional power supply current, and, eventually, because of possible damage of loudspeaker. By the way, if a watchdog is used, this situation can be circumvented, very probably...

I'm the fan of more sophisticated circuits. But if I would published it here, some guys would tell me, that there are much simpler ways to introduce this. Now, where I posted a simple solution, I'm asked, whether there are more sophisticated ones! You see my dilemma...

I would use an added 74HCMOS chip, where some outputs are connected together (anti-hogging resistors not to be forgotten!), and go via capacitor to loudspeaker. This would be more suited for my needs. But then you would need an additional chip.
Of course, it's possible to build a push-pull stage also by discretes. But then, due to rather low high state current of standard port of '51 micro an additional driver stage would be needed.

Kai