WPF AccessKeyManager annoying sound

I'm adding now some shortcut keys to my wpf application. I have one ComboBox and I want each item to be able to have a shortcut key. Now, I do this with custom class deriving from ComboBoxItem and having a property ShortcutKey, which register the access key via AccessKeyManager on set.

public class ComboBoxItemLanguage : ComboBoxItem
{
    private string _key;
    private ComboBox parent;

    public string LanguageCode
    {
        get; set;
    }

    public string ShortcutKey
    {
        get
        {
            return _key;
        }
        set
        {
            _key = value;

            AccessKeyManager.Register(value, this);
            AccessKeyManager.AddAccessKeyPressedHandler(this, AccessKeyPressedEventHandler);
        }
    }

    public ComboBoxItemLanguage()
    {

    }

    private void AccessKeyPressedEventHandler(object sender, AccessKeyPressedEventArgs e)
    {
        parent = parent ?? ItemsControl.ItemsControlFromItemContainer(this) as ComboBox; // gets the parent ComboBox
        parent.SelectedIndex = parent.Items.IndexOf(this);

        e.Handled = true;
    }
}

Then, in XAML it looks like this:

<ComboBox x:Name="cb_lang" SelectedValuePath="LanguageCode" SelectedValue="{Binding InputLanguage, UpdateSourceTrigger=PropertyChanged, Mode=OneWayToSource}"><local:ComboBoxItemLanguage Content="English" LanguageCode="en" ShortcutKey="e"/></ComboBox>