I found a solution regarding MKVmerge and only getting sound but no video. It seems to be changes to the header compression in MKVToolnix v 4.2.0 that causes the problem. Solution:
1) Downgrade to prior MKVtoolnix version 4.1.1
2) Use MKVtoolnix 4.2.0, but set "Compression" to "None" for each track (video and audio). This can be found in the "General Options" tab after you load a file and select a track.
Maybe this could be a problem for more of you soon ad hopefuly there will be included in a future upgrade.
Se original post: http://www.mpcclub.com/forum/showthread.php?t=24821