Local media players won't play audio on movies I have a generic Android set top box. It works fine mostly, except it won't play audio for .avi, .mkv, mp4, etc. files. I can successfully hear audio on movies via NetFlix and YouTube, but if I download a movie, there's no audio.

I have tried multiple media players, including MX Player and VLC but neither works.

I haven't knowingly changed something to cause this. Any ideas what it could be?
Android ,mx player ,vlc do not have a full codec compatibility due to licensing problem ,I think the audio on your movies is an ac3 or dts codec it's a 5.1 audio system so it doesn't play natively on Android because Android use 2.0 stereo as I said it can't be played natively but there is a codec for MX player that can play such audio format  follow those steps :


Go to this thread on XDA 
Download the codec that matches your device's processor type (if you don't now which one to choose search for an app called phone info on the play store it will give you your device's type download the right codec )
Put the downloaded zip somewhere on your device in this example it's /sdcard/download/codec_arm_V7_neon
Open MX player and go to setting >> Decoder scroll all the way down and click custom Codec ,navigate to the downloaded zip location and choose it.
You may be asked to close and then reopen the MX player ,do so 
You are good to go now your device can read AC3 and dts sound format.