I have had music playing via I2S using Volumio on the BBB but one channel had some distortion.
Within the GUI I selected “Black” as the output in the Playback screen, I believe thats all that is required. Give it a try and let us know the results.
Having said that I am also aware there are issues with resampling to 48khz which are yet to be resolved but you should get something out via the I2S connections.