If you mean the default audio, try AviMuxGUI. http://www.afterdawn.com/software/video_software/video_encoders/avi-mux_gui.cfm
Ooo and wif idiot proof pictures BTW I ca find my way in well laided out GUIs...its the not so good ones I get annoyed and confused with
Not bad but doing 1 file at a time when you ahve 2X is a pain. THe gui is nice wish soemone would merge it with mkv extract(as in select the mkv file change the info to what you want send it to a batch file and do the rest even automate it with a simpel change default track/sub for all or bring in new subs and what not)and make it support ogm/avi and others that can have dual tracks in it.