Video but no sound, or sound but no video

I have a library with 20 files, MKV, MP4.

I’ve installed Plex for Android (5.5.0.227) on a TV Box (BBox Miami) with a Quad Core BG2Q Marvell CPU with Android 5.1.1.

Without transcoding (Direct Play), i have sound but NO video; with transcoding enabled, i have video but NO sound

Result: I can’t watch my videos on this Box on Plex.

I’ve tried to put one of the videos on a USB pen drive, and the video reads correctly on the same box

Those same videos read fine on a Browser (Chrome or Firefox), Plex for Windows and also on two mobile phones (iPhone 5S and Honor 5C).

Videos are 1080p, 720p, MP4 and MKV, H.264 or H.265.