Your video files have different audio codecs, sampling rate (kHz)... and you want to normalize it, maybe for merging.

Following command will work with all .avi files (can be changed) in a directory
Will create sub-directory "new" with resulting files.
Will use aac codec - well known
Will use casual sampling rate of 48kHz.
Will use mp4 extension which is kind of container so no video re-coding is needed.

First via terminal, go to directory with the video files. Then run following (after modification if needed):

Code:
mkdir new && for i in *.avi; do ffmpeg -i "$i" -c:v copy -c:a aac -ar 48000 "./new/${i%.*}.mp4"; done