Subtitles are in an external file. I can change the file, but can't find a 'no subtitles' option.
It's a bug in Audience and it's already reported on Launchpad: https://bugs.launchpad.net/audience/+bug/1362910
P.S. If you've wandered to this post and saw this bug, and it affects you - please click "this bug affects me" on a launchpad to let developers know about the IPU