Allah, the Blessed and Exalted, says: "And when the Qur'an is recited, listen to it attentively and remain silent." [Surah al-A'raf: 204]
Imam al-Shafi'i (may Allah Almighty have mercy on him) said in his earlier (old) opinion: "In our view, this ruling applies specifically to recitation that is heard."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q2885