When I am using Android Auto with my paired with headset I don’t get voice prompts in my headset.
If you're Sena is paired with your phone and your phone is cabled to your Boombox through the usb for android auto, I believe you need your Sena paired to the Boombox through either the Harley WHIM ($300) or a Sena FreeWire ($250). The audio is routed out through the 7 pin din which the freewire or WHIM then bluetooths to your headset. I'm in the same situation. Harley Davidson decided not to enable their boombox to wirelessly connect unless you pay through the nose for the right to.
I thought I had the same issue but turns out the Sena voice, while paired to my phone, was nearly inaudible at highway speeds. I can here it when I've got my phone volume all the way up, my Boom radio muted and at slower speeds. I can hear my intercom just fine at highway speeds but the Sena voice for my GPS or phone calls or anything is really low. Would love to find a solution for this if anyone has a suggestion. I don't need to hear my Boom so I've not gotten the WHIM or FreeWire.