<?php

$group = '{"title":"Title", "artist":"Artist", "cover":"img/1.png", "file":"./file/1.mp3"}';

$array = json_decode($group, true);
$result = [];

foreach($array as $key => $value) {
    $result[] = $key . ':' . $value;
}

$result = implode(',', $result);

echo '{' . $result . '}';