Thanks for your answer, Ian.
In the meantime I have tried more things, and infact the BASS_Init call already causes the distorted sound to be played.
Infact any following call to a BASS .NET function changes the frequency of the distortion somehow. It sounds a bit like a late 90s modem dialing up.
The output of Version and Info is:
Bass Version: 2040F00
Speakers=2, MinRate=0, MaxRate=0, DX=0, EAX=False
I am not sure about Pulse or Alsa, but I would assume its Alsa as its a fresh install of raspbian.
Here is my simple code but already the first line causes the distorted sound.
Bass.BASS_Init(-1, 44100, BASSInit.BASS_DEVICE_DEFAULT, IntPtr.Zero);
var stream = Bass.BASS_StreamCreateFile("Data/Sounds/sample.ogg", 0L, 0L, BASSFlag.BASS_DEFAULT);
Bass.BASS_ChannelPlay(stream, false);
The same 3 calls in the C version play fine on the same raspberrypi:
BASS_Init(-1, 44100, 0, NULL, NULL);
HSTREAM stream = BASS_StreamCreateFile(FALSE, "Data/Sounds/sample.ogg", 0, 0, 0);
BASS_ChannelPlay(stream, FALSE);