Someone

for ((i=0; i<${#IN_FILES[@]}; i++)); do 
    if ((CHAPTERS > 0)); then
        if [[ ${GROUP[i]} ]]; then
            while read line; do
                group_chapt_array=( "${group_chapt_array[@]}" "$(for p in $line; do echo -n $(format_seconds $p),; done)" )
            done <<< $( make_group_chapters "${GROUP_CHAPTERS[i]}")
        fi
    fi
done

# each element gets all lines of output rather than one line per element
# group_chapt_array[0] is 00:00:33.089,00:01:16.187,00:00:00.834,00:00:43.933,00:01:27.031,00:02:10.130,
# rather than:
# group_chapt_array[0] :  00:00:33.089,00:01:16.187,
# group_chapt_array[1] :  00:00:00.834,00:00:43.933,00:01:27.031,00:02:10.130,