result

bash-4.4# cat melted-playlist.txt | gawk -f melted-simple.awk
{"payload":["index":"0","filename":"audio-bbc.xml","in":"0 2255 2256 2256 25.00","out":"","length":"","calculated_length":"","fps":"0.000000"],["index":"1","filename":"afasdfasdfasdf","in":"0 2255 2256 2256 25.00","out":"","length":"","calculated_length":"","fps":"0.000000"],["index":"2","filename":"one two","in":"0 2255 2256 2256 25.00","out":"","length":"","calculated_length":"","fps":"0.000000"],["index":"3","filename":"one two three","in":"0 2255 2256 2256 25.00","out":"","length":"","calculated_length":"","fps":"0.000000"]}