sort list box observable collection

hi,

my listbox itemsource is "sourceList" - which is MultiSelectCollectionView<string>

i want my list box to be sorted using sort description.but we need to add property with the sort description like below

<CollectionViewSource.SortDescriptions>
<scm:SortDescriptionPropertyName="PropertName"/>
</CollectionViewSource.SortDescriptions>
but there is no such property exist in my item source as my item source itself is property.
so how to sort my list box using sort descrion in xaml it self