Since MediaCoder 0.7.3, multi-threaded version of MPlayer and MEncoder are included in the full pack  distribution (in the past only included in optimization packs). However, as the MT version of MPlayer/MEncoder is still experimental, its inclusion brings some problems with MediaCoder. Decoding of Some H.264 files (mainly MKV) is blocked sometimes. If you encounter this issue, a workaround is to disabling multi-threaded decoding. You can do this by clicking Source button on Video tab. A pop up dialog should display the options of MEncoder and clicking “Decoding threads” and put a value of 1 (meaning single thread) in the blank at the bottom of the dialog.