The VideoLAN Forums

Yes i faced this problem in VLC. but the problem is because of the .mkv file.
some .mkv file not encoded properly. but you should try other player .

In the full screen mode or normal screen mode you can change volume with the help of the "ctrl + Up arrow " for increase volume and "ctrl + down arrow" key for decreased volume.

This problem occurred because of corrupt media file .
and if in all .rmvb file facing same problem then update your VLC.