BC into Winders™ (aka, Windows 😀), then go into your Control Panel, and make sure your audio levels, output device, etc., are set correctly. I've seen more than one person with this issue, and it's because the Windows install defaulted to like HDMI audio out (and their monitors didn't have speakers).
I've done BC on two '12 Mac Minis, a '10 MBP, both '12 and '15 MBP 15" models, and in every case, I had properly functioning audio.
(Now on my '18 Mini I used a VM, which also has fully functioning audio)