Merging avi files

I have 3 avi files and all of them seem to be encoded with VBR audio. So I cant use virtualdub to combine them. What I have done is save each file without the audio, and then combine all three files os i get a large file with just video and no audio. I have also extracted the audio into 3 separate files. Now I need a way to merge the audio files. Any suggestions, and could you please give me the steps?

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Extract the audio, fix it (CBR) & then add it back into each file with VDubMod , disabling the the original audio, resave with video set to direct stream copy & then join them.