Hello solveigforum,
First of all welcome to our community!
Unfortunately at the present moment non of our products allow you to mux separate audio/video stream to AVI.
Nevertheless if then you are going to convert the file to MP4, for muxing you can use SolveigMM ASF Multimplexer to be available at the following link - http://solveigmm.com/?Products&id=ASFMuxer
You will mux your AV file WAV files to ASF, without re-encoding so no difference if your temporary file will be AVI or ASF before MP4 conversion.