I was running Mint Sarah and had recently upgraded to Mint Sonya and just noticed the Pulse developers decided to unnecessarily break Dolby AC3 support. Like all good Debian LTS releases, this will never be fixed by an update and you'll have to fix it yourself.

I found the solution here. Essentially just edit

/etc/pulse/daemon.conf

and uncomment

default-sample-channels

and set it to whatever is reasonable for you.

Now I can go back to watching movies and hearing sweet helicopter fly overs.