First, you should see if the eps in question have the same framerate and running time. If they do, that will simplify things a bit.

You will need a variety of apps for this, something for demuxing the audio from the mpgs and avis, like virtualdub, virtualdubmod, tmpgenc, dgindex.
Something to decode (and later encode) the audio (likely in mp2 and/or mp3 format), like lame, toolame or maybe winamp or foobar2000.

Since they're tv-rips (I assume?), most likely the audio will not match up completely without some editing, so you'll also need a multitrack sound-editor, like audacity.
Load both tracks in the editor and match the 'new' track up to the one you're replacing. You may need to make a few edits/cuts here and there. When you're done, you just need to save the edited track, encode it again and mux it back with the video.