This bug can be reproduced by unplugging all audio devices from the computer. Turn on subtitles, then you will see that they do not work. This is particularity bad as I use the subtitles to make sure nothing will kill me, and I normally play with my headphones plugged in to something else. Thanks!
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Thank you for your report!
We're actually already tracking this issue in MC-148293, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
Does this also happen without Optifine? Optifine is a mod, and we cannot support mods here on the bug tracker.