“Indeed, the most disagreeable of sounds is the braying of donkeys.”
[Surah Luqmān 19]

He commanded him to lower his voice, just as He commanded the believers to lower their gaze and commanded him to be moderate in his pace. All of this concerns what is within his control; whether the voice is pleasant or unpleasant has no bearing on it.

Allah Most High said:

“Indeed, most of those who call out to you from behind the chambers do not understand.”
[Surah al-Hujurāt 4]

And He said:

“Do not raise your voices above the voice of the Prophet, nor speak loudly to him.”
Surah al-Hujurāt 2

And He said:

“Indeed, those who lower their voices in the presence of the Messenger of Allah are those whose hearts Allah has tested for righteousness.”
Surah al-Hujurāt 3

In Sahīh al-Bukhārī, ʿAbd Allāh ibn ʿAmr relates the description of the Prophet, peace and blessings be upon him, in the Torah:

“He is neither harsh nor coarse, nor does he raise his voice in the marketplaces. He does not repay evil with evil, but pardons and forgives.”