I suspect it has more to do with speaker emulation than CPU timing.

The ECE build has a speaker patch with some improvements, so you might try the game there to see if the sounds are audible.

Thank you for your answer ripsaw8080!
I just tried DOSbox ECE with default settings and the sounds are missing also in this version.

Well, default settings are not quite right for this game. Use machine=cga and cycles=270 (or thereabouts).

I tried the settings you suggested, but the helicopter sounds are always missing.
Thanks anyway!

For the helicopter sound, the game alternates between square wave counter values of 1 and 2, which are not audible frequencies, but just the re-programming of the timer counter value apparently causes a subtle noise. I suppose it could be considered an undesirable artifact, like the "snow" effect seen on old CGA hardware, but the game is making use of it and DOSBox does not emulate it.

Thank you for the technical insight, ripsaw8080.
I love these old programming tricks!

This issue has been resolved in the DOSBox-X fork in a recent commit.
https://github.com/joncampbell123/dosbox-x/co … ab1a65e2edf6f2f

For now you do need to compile it yourself. Hopefully there will be a new binary release in the next days.

1		if (p->write_latch == 0)
2		{
3			if (p->bcd == false)
4				p->cntr = 0x10000;
5			else
6				p->cntr=9999;
7		
8		/* ========================================= BEGIN ===  */
9		}	
10		
11		else if (p->write_latch == 1 && p->mode == 3)
12		{ 
13			
14            if (p->bcd == false)
15                p->cntr=0x10001;
16            else
17                p->cntr=10000;			
18
19		/* =========================================== END ===  */
20		
21		} else p->cntr = p->write_latch;
22
23		if ((!p->new_mode) && (p->mode == 2) && (counter == 0)) {

Is this OK?
I Modifed and Compare ist with the Video. Now I hear the sounds.

Maybe. An alternate idea is:

1     if (p->bcd==true) BCD2BIN(p->write_latch);
2     if (p->write_state != 0) {
3+        if (p->mode == 3) p->write_latch &= 0xfffe;
4         if (p->write_latch == 0) {
5             if (p->bcd == false) p->cntr = 0x10000;
6             else p->cntr=9999;
7         } else p->cntr = p->write_latch;

Seems like what the hardware might do instead of making special cases, but has implications for limiting possible frequencies, so needs to be checked.

I tried DOSBox-X (dosbox-x-windows-20200502-170825-windows.zip) and the helicopter sound is there!
However, while the game is playing sounds there is a slight background hum.
Thanks to all.