~/movies/ MacHD$ mplayer mkvmerge -o video.mkv video.avi audio.mp3 
MPlayer dev-SVN-r24007-4.0.1 (C) 2000-2007 MPlayer Team
CPU: Intel(R) Core(TM)2 Duo CPU     T7700  @ 2.40GHz (Family: 6, Model: 15, Stepping: 10)
CPUflags:  MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled for x86 CPU with extensions: MMX MMX2 SSE SSE2
Option -o has been renamed to -vo (video-out), use -vo.