There is another round-about way. I am not sure it will work with all video types, but it definitely worked for me with an mpg video, and should work with most videos that are played in DirectShow.
1. Make a copy of the video, and rename it with an audio extension, such as mpa.
2. Import it into MC.
2a. Most likely, it will still be imported as video. Change Media Type from Video or whatever it is to Audio inside MC.
2b. Make sure the file type (mpa in my example) is played in JRiver audio engine (using DirectShow filters). This is important, as mpa type is normally lumped together with mp3 type, which is played in JRiver audio engine (using plugins).
2c. Make sure Source Filter selection is set to Automatic for the type (mpa) as your normal selection of Source Filter for the type may not be able to handle the "fake" audio file.
3. Tools > Advanced Tools > Convert Format... to convert it to, say mp3.
Now you have an mp3 file of the audio track of the original video.