After facing a major problem, an important step is done to proceed further in the development.
During testing one big problem occurred with the chosen hardware. The Shield and the USB interface didn’t work as planned. In combination with Lineage OS 15, the shield wasn’t recognizable for the Raspberry Pi. The selected USB interface was shown in the terminal as loudspeaker and microphone, but only the loudspeaker output was there. I tested this process with different apps, which use the microphone, but with not success. The next step was to change the operating system of the Raspberry Pi. There I tried an older version of Lineage OS for Raspberry and a normal Android Version of Android Pie. Both attempts brought no success. Another step to fix this problem was a direct USB microphone without an external USB soundcard, but there was also no positive result. The solution was then to change the USB interface, though it was shown in the USB device list of the Raspberry.
Now that receiving and transmitting sound works, the development of the onboard software and the creation of use cases are the next big steps.