Using FFMPEG the command would look something like this:
ffmpeg -i <sourceVideoFile> -i <sourceAudioFile1> -i <sourceAudioFile2> -map 0:0 -map 0:1 -map 1:0 -map 2:0 -c:v copy -c:a copy <outputVideoFile>
Basically you're taking the source video file and multiple audio inputs, then mapping multiple audio files to the output's audio stream 0. Using -c:v copy and -c:a copy, you arent doing any encoding, so your file will keep its original quality.
That command was taken from the link below, which has more instructions:
Edit: Forgot to mention the point about wanting to do it in batch. Use the link below to create a bat file in Windows that would target all the specific types like *.avi (or all types using .). You'd want to replace the ffmpeg portion of their bat file with the above. Since you have one audio file, try something like:
for %%a in ("*.avi") do ffmpeg -i "%%a" -i -map 0:0 -map 0:1 -map 1:0 -c:v copy -c:a copy "%%a"
You might want to back up your files and make a copy elsewhere before you process them, because the above will overwrite them I believe.
Link to that below: