“They heard the sound of Yahweh God walking around in the garden.” This is one of the times in Scripture that God came into human form to fellowship with His creation, in this case, with Adam and Eve. Anyone who has walked in the woods knows the “sound” (not “voice” as in some translations) of someone walking. There is a distinct rustling and crunching sound. Adam and Eve, who lived in the garden, recognized the sound of God walking in it and hid. The fact that Adam and Eve recognized the sound God made as He walked shows that He came regularly to the garden to fellowship with Adam and Eve.
The translation “walking around” comes from the fact that the Hebrew verb “walk” is a Hitpael participle and therefore means more than just “walk,” it means more like “walking around” (cp. NAB, NLT), “walking to and fro” (Rotherham); “walking up and down” (YLT), or “moving about” (NET). The Garden of Eden was a small plot of land, small enough for Adam and Eve to care for it and protect it by themselves, and it is a wonderful picture that God would show up in person and walk around in the garden He created.
[For more on God coming in the form of a human being, see commentary on Acts 7:55.]
“at the time of the evening breeze.” The Hebrew text is more literally, “at the ruach (spirit, wind) of the day.” In the evening in Israel as the sun goes down and the air cools there is a breeze (ruach, “wind”) that blows over the land and it is a wonderful time to relax after the heat of the day. So after a hard day tending and guarding the Garden of Eden (all the animals ate plants so the garden had to be guarded), Adam and Eve would relax in the cooling “breeze of the day” and God would come and fellowship with them.