I've experienced a DAB issue twice in quick succession. The first time it seemed to resolve itself, and yesterday, as it happened again, I decided to investigate a bit further.

The symptoms:
When selected DAB, all stations are displayed, but there is no sound. The is an additional entry at the bottom of the list indicating Ensemble 1, with a "no speaker" sign (see attached).
Rebooting MMI (long press on big button + top right + menu) does not clear it.
Every other source (including FM / LW / MW) works fine, which indicated to me it was less likely to be a reception issue.

SOLUTION:
I found a useful youtube video with someone having this problem on an A5. The solution is strangely simple and impossible to find...
REMOVE the USB charger from the 12V plug in the front central storage unit!
IMG_20170912_153418.jpgIMG_20170912_153507.jpg

Hope this helps!