Actually, 31:6 speaks about purchasing idle tales / narratives. Go back to the Arabic, the word that's used is "haadith" (حديث) which means 'narratives', 'tales' / 'stories'. This word is used 3-5 times and neve means music. The verse yu cited has music in brackets.
(13. He (Allah) has ordained for you the same religion which He ordained for Nuh, and that which We have revealed to you, and that which We ordained for Ibrahim, Musa and `Isa saying you should establish religion and make no divisions in it. Intolerable for the idolators is that to which you...