I know what you are asking is useful on its own, but Zoom support starting with the microphone muted, or even not joining the audio stream at all, check out the

I know what you are asking is useful on its own, but Zoom support starting with the microphone muted, or even not joining the audio stream at all, check out these options: Settings->Audio->Mute my microphone when joining a meeting Settings->Audio->Automatically join audio by computer when joining a meeting