Solved(2) - much more elegantly
Update! It’s more simple without killing the audio drivers for bcm2835. Just edit two lines in the /etc/mycroft/mycroft.conf file. If the usb -card ist registrated as card 1 (aplay -l) then write this:
“play_wav_cmdline”: “aplay -Dhw:1,0 %1”,
“play_mp3_cmdline”: “mpg123 -a hw:1,0 %1”,
and all sounds are routed to the usb sound card.
DEPRECATED: Solved! (With a big hammer 
)
Thank you very much for your detailed answer! Nevertheless I decided to go a different way for the following reason: pulseaudio worked correctly on operating system level. In the mycroft-setup-wizard the USB soundcard was recognized correctly. The audio test was successful, I could hear the word test, the volume control worked. The (USB-)microphone test seemed to be unsuccessful, because I couldn’t hear anything afterwards. But there was a current file /tmp/test.wav. This file could be played with paplay /tmp/test.wav. So it was proven that basically the sound was both output and recorded via USB. Only the assignment to which output the output is routed does not work neither in the wizard nor in the running Picroft program.
How did I solve it:
I noticed in voice.log that Picroft always tries to address the (alsa) device 0. But this is the bcm2835 chip.
First I deactivated the audio chip driver in raspberry:
sudo nano /etc/modprobe.d/alsa-blacklist.conf
I added the line blacklist snd_bcm2835
This disables the output via the 3.5 mm connector. However, this alone was not enough. I had to intervene much deeper into the system:
sudo nano /lib/modprobe.d/aliases.conf
and there comment out the line options snd-usb-audio index=-2
This causes the USB device to get priority.
Then reboot. Now I could communicate with Mycroft on a Raspberry Pi 4 via the USB soundcard.
But what I don’t hear is the confirmation sound after calling “Hey Mycroft”. This is still a bit strange.
The solution is, I think, quite brutal, but it works. It seems to me that the wizard still needs to be revised. Now that the hardware is working as desired, I can finally start to explore the functionalities: What I already noticed: The system is quite slow. I have been using SNIPS so far. This is much faster (on a Raspberry Pi 3!). But that is another matter…