My clip has no sound

Some engines are silent, and on others the soundtrack is a switch you may have left off.

Hailuo 3 Max, Runway Gen-4.5 and Grok Imagine render no sound. Pick another engine for a clip with audio.
On Veo and Kling the soundtrack is optional and billed per second: check the Soundtrack switch in Settings before rendering.
A Replace result is silent by design; "With original sound" lays the source's audio under it.
For narration, render the voice line separately in Voice and lay it under the cut in Cinema or your editor.