LobbyVoice.cs

var lobbyManager = discord.GetLobbyManager();
lobbyManager.ConnectLobby(lobbyId, lobbySecret, (Discord.Result result, ref Discord.Lobby lobby) =>
{
    if (result == Discord.Result.Ok)
    {
        // User is in the lobby
        lobbyManager.ConnectVoice(lobby.Id, (res) =>
        {
            if (res == Discord.Result.Ok)
            {
                // User is connected to voice
            }
        });
    }
});