It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) recited in prayer at night, he would sometimes raise his voice and sometimes lower it."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4767
Hadith Gradingالألبانی:حسن
Hadith Takhrij[ابن نصر] عن أبي هريرة. صحيح أبي داود ١١٩٩: د، حب، ك.