A minimalistic/simple music player in a Discord voice channel, using the "eris" NodeJS Discord wrapper. Put this into your existing Eris bot.

discord_music.js

// Plays music in a Discord voice channel, using the "eris" NodeJS Discord wrapper.
// Requires ffmpeg (https://github.com/abalabahaha/eris/issues/470).

const ytdl = require('ytdl-core')

const VOICE_CHANNEL_ID = "VOICE_CHANNEL_ID_HERE"
const YOUTUBE_VIDEO = "http://www.youtube.com/watch?v=5qap5aO4i9A"

function joinAndPlay() {
    bot.joinVoiceChannel(VOICE_CHANNEL_ID).catch((err) => {
        console.error("Error joining vc: " + err.message)
    }).then((connection) => {
        connection.play(ytdl(YOUTUBE_VIDEO, { audioonly: true }))
        connection.once("end", joinAndPlay)
        connection.once("error", joinAndPlay)
    })
}

bot.on('ready', (evt) => {
    joinAndPlay()
})