Imam al-Shafi'i (may Allah have mercy on him) said: "Indeed Allah, the Mighty and Majestic, says: 'And do not recite your prayer too loudly, nor too quietly.' [Surah al-Isra: 110]
What is meant by it (the prayer) is supplication, and Allah knows best.
And the meaning of 'Do not be loud' is that you should not raise your voice very high, and the meaning of 'nor too quiet' is that it should not be so quiet that you yourself cannot hear it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q3008