I haven't had problems with it for a while but when I did this method worked for me:
Go to your sound settings
Select audio output settings
Switch from HDMI to optical digital (even if you use HDMI)
Select every setting there is, then SAVE IT.
Once its saved, it will take you back to sound settings, select audio multi-output and turn it ON.
I also just read that it needs an internet connection to classify the audio, so if you disconnect from the net before you play the file that may work. If it has already noted that a file is DRM protected it won't work though, but it has been suggested that you can disconnect from the net, rename the file and then try again with the renamed file.
I have't tried this method though so it may be bullshit.