Nostrum.Voice.join_channel

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

join_channel(guild_id, channel_id, self_mute \\ false, self_deaf \\ false)

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Specs

Joins or moves the bot to a voice channel.

This function is equivalent to Nostrum.Api.update_voice_state/4.