@fast_api.handler(...)
    #fast api args to volume() func
    #...

    @bot.tree.command(name="volume", description="Set the volume of the player")
    @app_commands.describe(volume="Set the volume to a number between 0 and 100")
    async def volume(interaction: discord.Interaction, volume: int):
        await lavalink.volume(interaction.guild.id, volume)
        await interaction.response.send_message(f"Set the volume to {volume}%.")
    return volume(...)