Since the recent update to MX Player, many users are running into a new problem: there's no sound in certain videos. It turns out the latest release of the popular video player removed support for two audio codecs: AC3 and MLP. Unfortunately, it seems this is a result of licensing issues, meaning MX Player will no longer ship with built-in support to play these audio formats. However, there is a simple workaround that will get things working again with relatively little hassle.
Restoring functionality is pretty simple, it only involves downloading a custom-built codec and pointing MX Player to the right file. Instructions and download links are available in this thread on XDA. There are several different architectures supported, and each has been broken up into individual files. The architecture for each device can be found by opening MX Player and going to the overflow menu -> Settings -> Decoder, and scrolling down to the bottom where the Download custom codec item details the correct type. Alternatively, there is an "AIO Pack" available with all of the different architectures bundled together, but it is a larger download. Place the file on internal storage, and tap on Custom codec (also in the Decoder screen) to point MX Player to the plugin. After a restart, the codec should be installed and working. To confirm, just go to the overflow menu -> Help -> About, and look for the "Custom codec" line near the top.
While these extra steps aren't quite as convenient as a simple download from the Play Store, it's not too much trouble to get things back in working order. Let's just hope this is a fairly isolated incident and other video players won't have to make similar compromises.