tap on bus 0 of AVAudioEngines mainMixerNode gives buffer with 0s

Hi,

by utilizing the AVAudioEngine the application on a Mac is supposed to process the audio data of the device output.

the following code:

audioDataSource.audioEngine.mainMixerNode.installTap(onBus: 0, bufferSize: 4096, format: nil) { (buffer, time) in
            self.processAudio(buffer: buffer)
} 

gives me a buffer filled with 0s

when the application starts the console output is:

2021-09-04 20:20:37.469550+0200 av01[1866:65350] [plugin] AddInstanceForFactory: No factory registered for id <CFUUID 0x600001488600> F8BB1C28-BAE8-11D6-9C31-00039315CD46

2021-09-04 20:20:37.528539+0200 av01[1866:65350]   no saved enable hardware sample rate converter preference found

i am running: macOS Big Sur Xcode Version 12.5.1