[Trisquel-users] Free software "muxer"

[gnuser]

Hello everyone.
I have a video in FLV format, that contain H264 codec and AAC audio. I would  
like to "remux" (I think that is the correct term) into a MKV. I don't want  
to convert, because it already has the codecs that I need.
Is there any free software tool (preferably in the reps, and with a GUI but I  
will take console if needed) to do this?