#ffmpeg -f concat -safe 0 -i <(grep -o . <msg.txt | while read ch; do echo "file 'mp3/${ch^^}.mp3'"; done) -c copy output.mp3

cat msg.txt |  sed 's/./\L&/g' | sed ':a;N;$!ba;s/\n/~/g' | sed 's/ /_/g' | grep -o . | while read ch; do echo "file 'mp3/$ch.mp3'"; done >list.txt
ffmpeg -f concat -safe 0 -i list.txt -c copy -y msg.mp3
rm -f list.txt