How do I download/save a backup of OSX's "Enhanced Quality" system voices for future use?
Search for CatalogVersion 2 ApplePostURL http://swpost.apple.com in a browser. Or download one or several of the Apple update catalog files directly e.g:


10.9 catalog
10.10 catalog
10.11 catalog


Open the file(s) with a decent text editor. Search the file for MLV_locale_name with locale: your preferred locale and name: the voice name.

E.g. searching for MLV_it_IT_alice will reveal:

        ...
        &lt;key&gt;031-11791&lt;/key&gt;
        &lt;dict&gt;
            &lt;key&gt;ServerMetadataURL&lt;/key&gt;
            &lt;string&gt;http://swcdn.apple.com/content/downloads/44/20/031-11791/gm3k0mrgb2nhbw6nmomxgshi9cd36lv9ke/MLV_it_IT_alice.smd&lt;/string&gt;
            &lt;key&gt;Packages&lt;/key&gt;
            &lt;array&gt;
                &lt;dict&gt;
                    &lt;key&gt;Size&lt;/key&gt;
                    &lt;integer&gt;264300527&lt;/integer&gt;
                    &lt;key&gt;MetadataURL&lt;/key&gt;
                    &lt;string&gt;https://swdist.apple.com/content/downloads/44/20/031-11791/gm3k0mrgb2nhbw6nmomxgshi9cd36lv9ke/MLV_it_IT_alice.pkm&lt;/string&gt;
                    &lt;key&gt;URL&lt;/key&gt;
                    &lt;string&gt;http://swcdn.apple.com/content/downloads/44/20/031-11791/gm3k0mrgb2nhbw6nmomxgshi9cd36lv9ke/MLV_it_IT_alice.pkg&lt;/string&gt;
                &lt;/dict&gt;
            &lt;/array&gt;
            &lt;key&gt;PostDate&lt;/key&gt;
            &lt;date&gt;2015-01-22T18:33:08Z&lt;/date&gt;
            &lt;key&gt;Distributions&lt;/key&gt;
            &lt;dict&gt;
                &lt;key&gt;English&lt;/key&gt;
                &lt;string&gt;https://swdist.apple.com/content/downloads/44/20/031-11791/gm3k0mrgb2nhbw6nmomxgshi9cd36lv9ke/MLV_it_IT_alice.dst/031-11791.English.dist&lt;/string&gt;
            &lt;/dict&gt;
        &lt;/dict&gt;
        ...


containing the ServerMetadataURL and the direct pkg download URL: MLV_it_IT_alice.pkg

Enter the pkg download URLs in a browser or a download manager to get all desired voices.