I am working on creating a Windows Phone app that will play a series of sound clips selected from a list. I am using the MVVM (Model View View-Model) Design pattern and have designed a model for my data, along with a view model for my page. Here is what the XAML for the ListBox looks like:

<ListBox x:Name="MediaListBox" Margin="0,0,-12,0" ItemsSource="{Binding Media}" SelectionChanged="MediaListBox_SelectionChanged" HorizontalContentAlignment="Stretch" VerticalContentAlignment="Stretch">

                <ListBox.ItemTemplate >
                    <DataTemplate>
                        <StackPanel Margin="0,0,0,17" Width="432" Orientation="Horizontal">
                            <Image Source="../Media/Images/play.png" />
                            <StackPanel >
                                <TextBlock Text="{Binding Title}" TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
                                <TextBlock Text="{Binding ShortDescription}" TextWrapping="Wrap" Margin="12,-6,12,0" Visibility="{Binding ShortDescriptionVisibility}" Style="{StaticResource PhoneTextSubtleStyle}"/>
                                <TextBlock Text="{Binding LongDescription}" TextWrapping="Wrap" Visibility="{Binding LongDescriptionVisibility}" />
                                <StackPanel>
                                    <Slider HorizontalContentAlignment="Stretch" VerticalContentAlignment="Stretch" Visibility="{Binding LongDescriptionVisibility}" ValueChanged="Slider_ValueChanged" LargeChange="0.25" SmallChange="0.05" />
                                </StackPanel>
                            </StackPanel>
                        </StackPanel>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>

My question is this: I want to be able to expand and collapse part of the items in the ListBox. As you can see, I have a binding for the visibility. That binding is coming from the MediaModel. However, when I change this property in the ObservableCollection, the page is not updated to reflect this.

The ViewModel for this page looks like this:

public class ListenPageViewModel : INotifyPropertyChanged
{
    public ListenPageViewModel()
    {
        this.Media = new ObservableCollection<MediaModel>;

    }

    /// <summary>
    /// A collection for MediaModel objects.
    /// </summary>
    public ObservableCollection<MediaModel> Media { get; private set; }

    public bool IsDataLoaded { get; private set; }

    /// <summary>
    /// Creates and adds the media to their respective collections.
    /// </summary>
    public void LoadData()
    {
        this.Media.Clear();
        this.Media.Add(new MediaModel()
        {
            Title = "Media 1",
            ShortDescription = "Short here.",
            LongDescription = "Long here.",
            MediaSource = "/Media/test.mp3",
            LongDescriptionVisibility = Visibility.Collapsed,
            ShortDescriptionVisibility = Visibility.Visible
        });

        this.Media.Add(new MediaModel()
        {
            Title = "Media 2",
            ShortDescription = "Short here.",
            LongDescription = "Long here.",
            MediaSource = "/Media/test2.mp3",
            LongDescriptionVisibility = Visibility.Collapsed,
            ShortDescriptionVisibility = Visibility.Visible
        });

        this.IsDataLoaded = true;
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(String propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (null != handler)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

The bindings work correctly and I am seeing the data displayed; however, when I change the properties, the list does not update. I believe that this may be because when I change things inside the observable collection, the property changed event is not firing.

What can I do to remedy this? I have poked around for some info on this, but many of the tutorials don't cover this kind of behavior. Any help would be greatly appreciated!

Thanks

Edit: As requested, I have added the MediaModel code:

    public class MediaModel : INotifyPropertyChanged
{
    public string Title { get; set; }
    public string ShortDescription { get; set; }
    public string LongDescription { get; set; }
    public string MediaSource { get; set; }
    public Visibility LongDescriptionVisibility { get; set; }
    public Visibility ShortDescriptionVisibility { get; set; }

    public MediaModel()
    {
    }

    public MediaModel(string Title, string ShortDescription, string LongDescription, string MediaSource, Visibility LongDescriptionVisibility, Visibility ShortDescriptionVisibility)
    {
        this.Title = Title;
        this.ShortDescription = ShortDescription;
        this.LongDescription = LongDescription;
        this.MediaSource = MediaSource;
        this.LongDescriptionVisibility = LongDescriptionVisibility;
        this.ShortDescriptionVisibility = ShortDescriptionVisibility;
    }
    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(String propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (null != handler)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

Originally, I did not have this class implement the INotifyPropertyChanged. I did this to see if it would solve the problem. I was hoping this could just be a data object.