$ ffmpeg -i videoM.mkv -i audio.aac -acodec copy -vcodec copy videoC.mkv
ffmpeg version v0.7-1698-g2a6eb06, Copyright (c) 2000-2011 the Libav developers
  built on Nov  3 2011 22:28:55 with gcc 4.6.2
This program is not developed anymore and is only provided for compatibility. Use avconv instead (see Changelog for the list of incompatible changes).
[matroska,webm @ 0x1883780] Estimating duration from bitrate, this may be inaccurate

Seems stream 0 codec frame rate differs from container frame rate: 180000.00 (180000/1) -> 29.97 (30000/1001)
Input #0, matroska,webm, from 'videoM.mkv':
  Duration: 00:05:49.63, start: 0.000000, bitrate: N/A
    Stream #0.0(eng): Video: h264 (High 10), yuv420p10le, 1280x720 [PAR 1:1 DAR 16:9], 29.97 tbr, 20k tbn, 180k tbc (default)
[aac @ 0x1886360] max_analyze_duration reached
[aac @ 0x1886360] Estimating duration from bitrate, this may be inaccurate
Input #1, aac, from 'audio.aac':
  Duration: 00:05:52.13, bitrate: 127 kb/s
    Stream #1.0: Audio: aac, 44100 Hz, stereo, s16, 127 kb/s
File 'videoC.mkv' already exists. Overwrite ? [y/N] y
Output #0, matroska, to 'videoC.mkv':
  Metadata:
    encoder         : Lavf53.10.0
    Stream #0.0(eng): Video: libx264, yuv420p10le, 1280x720 [PAR 1:1 DAR 16:9], q=2-31, 1k tbn, 20k tbc (default)
    Stream #0.1: Audio: libfaac, 44100 Hz, stereo, 127 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #1.0 -> #0.1
Press ctrl-c to stop encoding
[matroska @ 0x18d06c0] Application provided invalid, non monotonically increasing dts to muxer in stream 0: 47948 >= 47948
av_interleaved_write_frame(): Invalid argument