MKV to AVI under Linux

You need:
MKVtoolnix

1. MKV is a container, to see what the file contains:
$ mkvinfo movie.mkv
Ex:
+ EBML head
|+ Doc type: matroska
|+ Doc type version: 1
|+ Doc type read version: 1
+ Segment, size 736993124
|+ Seek head (subentries will be skipped)
|+ EbmlVoid (size: 4027)
|+ Segment information
| + Timecode scale: 1000000
| + Muxing application: libebml v0.7.3 + libmatroska v0.7.4
| + Writing application: mkvmerge v1.0 ('Soul Food To Go') built on Nov 17 2004 15:06:35
| + Duration: 3682.987s (01:01:22.987000000)
| + Date: Wed Dec 8 15:42:45 2004 UTC
| + Title: Il Coraggioso - The Brave
| + Segment UID: 0x84 0xad 0xc1 0x0b 0x8e 0x9b 0x06 0x84 0xef 0x2a 0xf9 0xa6 0xf4 0x94 0x07 0x5d
| + Next segment UID: 0xeb 0x46 0x75 0xb4 0x7a 0x71 0xc2 0x7f 0xc7 0x36 0x44 0x06 0x9b 0xe4 0x90 0x76
|+ Segment tracks
| + A track
| + Track number: 1
| + Track UID: 38700354
| + Track type: video
| + Default flag: 1
| + Lacing flag: 0
| + MinCache: 1
| + Timecode scale: 1.000000
| + Codec ID: V_MS/VFW/FOURCC
| + CodecPrivate, length 40 (FourCC: XVID, 0x44495658)
| + Default duration: 40.000ms (25.000 fps for a video track)
| + Language: und
| + Name: Il Coraggioso - the Brave
| + Video track
| + Pixel width: 608
| + Pixel height: 336
| + Display width: 608
| + Display height: 336
| + A track
| + Track number: 2
| + Track UID: 3348909422
| + Track type: audio
| + Default flag: 1
| + Lacing flag: 1
| + MinCache: 0
| + Timecode scale: 1.000000
| + Codec ID: A_AAC/MPEG2/LC/SBR
| + Default duration: 42.667ms (23.438 fps for a video track)
| + Language: ita
| + Name: Ita aac 5.1
| + Audio track
| + Sampling frequency: 24000.000000
| + Channels: 6
| + Output sampling frequency: 48000.000000
|+ EbmlVoid (size: 1024)
|+ Cluster

2. select the numbers of the trucks you want to extract and type:
$ mkvextract tracks "/mnt/common/Film/movie.mkv" 1:/mnt/common/Film/movie.avi 2:/mnt/common/Film/movie.aac

3. If the audio file has the aac format you need also:
faad
lame

4. To convert the file audio in mp3 format:
$ faad -d /mnt/common/Film/movie.aac
$ lame /mnt/common/Film/movie.wav

5. Let's put all toghether:
$ mencoder -audiofile movie.wav.mp3 -oac mp3lame -ovc copy movie.avi -o movie_complete.avi