Thanks Colin!
This means that the I2C protocol running on pins P9_19 and P9_20 (19 SCL, 20 SDA) is required to complete before the BBB will send audio data on P9_28 (AUD_DOUT).
Without a cape, I don’t see how I can get audio data… perhaps I can use an Arduino and the Wire.h library to act as an I2C slave and that may do it.
Or wait 51 more days for the Audio Capes to be back in stock.
In any case, thanks again!