I recorded my own file and transmitted it. The result was such that the audio speed was drastically reduced and there was a broad noise on it. I don't know how to find out the correct properties of the audio file. But then there is a forum for rpitx.
It is a matter of time until i or someone else finds the solution. BTW: The transmitted file is sampleaudio.wav . First, there is a carrier for a few seconds, then sampleaudio.wav is transmitted.