Multi-thread console text animations with std::cout

I'm trying to create a function that can animate multiple strings to the console at the same time. By "animate," I mean print a character, wait a specified amount of time, then print the next character, and so on.

This is what I've tried so far:

/**
 @param msg        Message to animate
 @param sleep_time Time to wait between each letter
 @param wait       Whether or not to wait for the current thread to join before returning
*/
void animate(const std::string& msg, const unsigned long long sleep_time, const bool wait = true)
{
    const std::atomic<std::chrono::milliseconds> t_sleep_time =
        std::chrono::milliseconds(sleep_time);

    std::stringstream msg_strm;
    msg_strm << msg;

    std::thread animate_thread([&msg_strm, &t_sleep_time]() -> void
    {
        char letter;

        while ((letter = msg_strm.get()) != EOF)
        {
            std::cout << letter << std::flush;
            std::this_thread::sleep_for(t_sleep_time.load());
        }

        return;
    });

    if (wait)
    {
        animate_thread.join();
    }
    else
    {
        animate_thread.detach();
    }
}

This is the driver code for it:

int main()
{
    animate("Hello", 500, false);
    std::cout << '\n' << std::endl;
    animate("Welcome", 400, true);
    std::cout << "\n\nEnd" << std::endl;
}

And this is the output ("Wecome" animates as sluggishly):

Welcome

End

What happened to "Hello"? I'm very new to multi-threading, so a detailed explanation would be very much appreciated. Ideally, what I'd like to happen, is to have "Hello" animating on one line and "Welcome" on the next. Is this possible?