成 is an uncommon surname (around #115). 乐 as a given-name character is usually read lè ('joy') in modern names, but yuè ('music') in classical compounds. Without additional context, default to Chéng Lè for a modern personal name.
history
In classical sources, 成乐 can appear as a verb phrase 'to complete music / bring the performance to its conclusion' - e.g. 《礼记·乐记》 uses 乐 in its ritual-music sense.
