Do you know any program that converts mkv to avi format?

You may or may not be able to do it. Depends on the video in the mkv container. One of things mkv allows for is variable framerate video, IE the video can change frame rates in the middle of the file. If this is the case, it is damn near impossible to convert.

I did it with imkvextract and ffmpeg....

Well I don't know for sure whether it formats all the VMK formats but it sure recognizes almost every kind of video format and converts it to almost everything at least most common known video formats. The program that I am using is called total video convertor. It is good because it's free and downloadable.