Isso funciona - extensão movida fora das aspas, pois -name
flag permite:
#!/bin/bash
# $1 -- extension, like MP4
# $2 -- output file name
echo Extension: $1
echo Output: $2
echo Files:
find . -name \*.$1 -printf "file '$PWD/%p'\n" | sort
ffmpeg -f concat -i <(find . -name \*.$1 -printf "file '$PWD/%p'\n" | sort) -c copy $2