D.Net 1.0 audio example

AudioModule.cs

using System.Threading.Tasks;
using Discord.Commands;

public class AudioModule : ModuleBase<ICommandContext>
{
    // Scroll down further for the AudioService.
    // Like, way down
    private readonly AudioService _service;

    // Remember to add an instance of the AudioService
    // to your IServiceCollection when you initialize your bot
    public AudioModule(AudioService service)
    {
        _service = service;
    }

    // You *MUST* mark these commands with 'RunMode.Async'
    // otherwise the bot will not respond until the Task times out.
    [Command("join", RunMode = RunMode.Async)]
    public async Task JoinCmd()
    {
        await _service.JoinAudio(Context.Guild, (Context.User as IVoiceState).VoiceChannel);
    }

    // Remember to add preconditions to your commands,
    // this is merely the minimal amount necessary.
    // Adding more commands of your own is also encouraged.
    [Command("leave", RunMode = RunMode.Async)]
    public async Task LeaveCmd()
    {
        await _service.LeaveAudio(Context.Guild);
    }
    
    [Command("play", RunMode = RunMode.Async)]
    public async Task PlayCmd([Remainder] string song)
    {
        await _service.SendAudioAsync(Context.Guild, Context.Channel, song);
    }
}

AudioService.cs

using System.Collections.Concurrent;
using System.Diagnostics;
using System.IO;
using System.Threading.Tasks;
using Discord;
using Discord.Audio;

public class AudioService
{
    private readonly ConcurrentDictionary<ulong, IAudioClient> ConnectedChannels = new ConcurrentDictionary<ulong, IAudioClient>();

    public async Task JoinAudio(IGuild guild, IVoiceChannel target)
    {
        IAudioClient client;
        if (ConnectedChannels.TryGetValue(guild.Id, out client))
        {
            return;
        }
        if (target.Guild.Id != guild.Id)
        {
            return;
        }

        var audioClient = await target.ConnectAsync();

        if (ConnectedChannels.TryAdd(guild.Id, audioClient))
        {
            // If you add a method to log happenings from this service,
            // you can uncomment these commented lines to make use of that.
            //await Log(LogSeverity.Info, $"Connected to voice on {guild.Name}.");
        }
    }

    public async Task LeaveAudio(IGuild guild)
    {
        IAudioClient client;
        if (ConnectedChannels.TryRemove(guild.Id, out client))
        {
            await client.StopAsync();
            //await Log(LogSeverity.Info, $"Disconnected from voice on {guild.Name}.");
        }
    }
    
    public async Task SendAudioAsync(IGuild guild, IMessageChannel channel, string path)
    {
        // Your task: Get a full path to the file if the value of 'path' is only a filename.
        if (!File.Exists(path))
        {
            await channel.SendMessageAsync("File does not exist.");
            return;
        }
        IAudioClient client;
        if (ConnectedChannels.TryGetValue(guild.Id, out client))
        {
            //await Log(LogSeverity.Debug, $"Starting playback of {path} in {guild.Name}");
            using (var ffmpeg = CreateProcess(path))
            using (var stream = client.CreatePCMStream(AudioApplication.Music))
            {
                try { await ffmpeg.StandardOutput.BaseStream.CopyToAsync(stream); }
                finally { await stream.FlushAsync(); }
            }
        }
    }

    private Process CreateProcess(string path)
    {
        return Process.Start(new ProcessStartInfo
        {
            FileName = "ffmpeg.exe",
            Arguments = $"-hide_banner -loglevel panic -i \"{path}\" -ac 2 -f s16le -ar 48000 pipe:1",
            UseShellExecute = false,
            RedirectStandardOutput = true
        });
    }
}