[وَاقْصِدْ فِى مَشْيِكَ]

(And be moderate in your walking,) means, walk in a moderate manner, neither slow and lazy nor excessively fast, but be moderate, somewhere in between these two extremes.

[وَاغْضُضْ مِن صَوْتِكَ]

(and lower your voice.) means, do not exaggerate in your speaking and do not raise your voice unnecessarily. Allah says:

[إِنَّ أَنكَرَ الاٌّصْوَتِ لَصَوْتُ الْحَمِيرِ]

(Verily, the harshest of all voices is the braying of the asses.) Mujahid and others said, "The most ugly of voices is the voice of the donkey, i.e., when a person raises his voice, the resulting noise is like the voice of a donkey in its loudness. Moreover this is hateful to Allah. Likening a loud voice to that of a donkey implies that it is forbidden and extremely blameworthy, because the Messenger of Allah said:

«لَيْسَ لَنَا مَثَلُ السُّوءِ، الْعَائِدُ فِي هِبَتِهِ كَالْكَلْبِ يَقِيءُ ثُمَّ يَعُودُ فِي قَيْئِه»

(It is not befitting us to be an evil parable. The person who takes back his gift, he is like a dog that vomits and then goes back to his vomit.)