I don't know if RecordMyDesktop can do this (I doubt it, since it's probably not a common need). You can split the file with ffmpeg, though:
## Get the audio:
ffmpeg -i input.ogv -vn -c copy audio.ogg
## Get the video:
ffmpeg -i input.ogv -an -c copy video.ogv
## Get both with a single command:
ffmpeg -i input.ogv -vn -c copy audio.ogg -an -c copy video.ogv
If you want the audio in an MP4, you'll want to re-encode it to AAC (AAC is the actual audio codec, while MP4 is a 'container' format, which can contain many different video, audio and subtitle streams). See the AAC encoding guide on the ffmpeg wiki. That wiki also contains useful guides to video encoding - check out the x264 guide if you want to put the video in an MP4.