Sound is played but it shouldn't

I just configured my system to use the voice speech by Deskbot (PCLC 5.00d). Since I havn't wanted to hear any sound up to now, I simply had the speaker switched off but havn't done anything on the settings.
So the first thing I had to do now, was to disable the sounds and to enable the voices. Unfortunately I still hear the sounds, e.g. countdown, R2D2 etc. although I put blank fields everywhere - for all drivers and in the options/sound tab. I also closed and restarted PCLC. For Info - I do not yet use the special tags liek {laptime}to play the voices. All sound fields are really empty.

Was this the right approach and it is a bug - or do I have to do something different?

This is not a bug, countdown sound will be always played (at least if your countdown > 0).
R2D2 is played when PCLC reject a lap (PCLC display the reason of the rejected lap in message box to know).
Another system sound is also played when lap time is bigger than the maximum lap time (more information here https://pclapcounter.forumotion.com/pc-lap-counter-general-f1/security-and-security2wav-t97.htm

Ah yes - got it.

Would it help to replace these two sound files by blank ones?

Yes , you can replace these 2 sound files with empty one (wav which content silence).