USB Audio SOF

I'm using SDK 15.2 and trying to set up a USB mic class. I am having trouble getting the SOF audio interrupt to fire.

I initialize the app_usbd module with the configuration for enable_sof set to true. I register an sof event handler with app_usbd_audio_sof_interrupt_register() like in the SDK USB Audio example which sets it up for headphones. The SOF event never triggers my sof event handler for the microphone. Instead, I do get SOF events inside usbd user event handler where I can start audio transmission. I have tried this with APP_USBD_CONFIG_SOF_HANDLING_MODE set to 1 (compress queue) as well as 2 (Interrupt). 

Am I missing some configuration?