Cela semble incroyable, mais je l'ai vérifié !
Lancez GarageBand, quittez... Quicktime et les autres sont revenus !

Thanks Coolhive! That worked for me as well! For some reason all audio stopped working in Quicktime. A re-install over the existing package didn't help. iMovie was kind enough to warn me that audio wasn't going to work, QT player just played the movie without sound. Couldn't get any movies to play in either Firefox or Safari. Other movie players worked fine. But I ran Garageband and the problem was fixed. Very strange.

Apple - Discussions - No sound in Quicktime