Nostrum.Voice.set_is_speaking

You're seeing just the function set_is_speaking, go back to Nostrum.Voice module for more information.
Link to this function

set_is_speaking(guild_id, speaking)

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Specs

set_is_speaking(Nostrum.Struct.Guild.id(), boolean()) :: :ok

Low-level. Set speaking flag in voice channel.

This function does not need to be called unless you are sending audio frames directly using Nostrum.Voice.send_frames/2.