Instead of returning "music/..." (like it does "xml/..." and as it should if these are relative URLs), the skin 
returns "/music/...". The resulting final url is "http://blabla//music/..." which is not interpreted 
correctly by the server (rightly so) and returns 404.
To cut a long story short, here's a patch.

Index: server/HTML/xmlTelCanto/browsedb.html
===============================================================
====
--- server/HTML/xmlTelCanto/browsedb.html       (revision 3129)
+++ server/HTML/xmlTelCanto/browsedb.html       (working copy)
@@ -7,6 +7,6 @@
        [% IF warn %]<still_scanning/>[% END %]
        [% IF pwd_list %]<location>[% pwd_list %]</location>[% END %]
        [% IF browselist_header %][% browselist_header %][% END %]
-       [% IF coverArt %]<coverart><![CDATA[/music/[% coverArt %]/cover.jpg]]></coverart>[% END %]
+       [% IF coverArt %]<coverart><![CDATA[music/[% coverArt %]/cover.jpg]]></coverart>[% END %]
        [% browse_list %]
 </browse>
Index: server/HTML/xmlTelCanto/_xml_song_full.html
===============================================================
====
--- server/HTML/xmlTelCanto/_xml_song_full.html (revision 3129)
+++ server/HTML/xmlTelCanto/_xml_song_full.html (working copy)
@@ -45,7 +45,7 @@
 
        [% IF track.url %]<path>[% track.url | unuri | utf8decode | html %]</path>[% END %]
 
-       [% IF track.coverArt('cover', 1) %]<coverart><![CDATA[/music/[% track.id %]/cover.jpg]]></
coverart>[% END %]
+       [% IF track.coverArt('cover', 1) %]<coverart><![CDATA[music/[% track.id %]/cover.jpg]]></
coverart>[% END %]
 
        [% IF track.moodlogic_mixable %]<moodlogic_mixable>[% track.moodlogic_mixable | html %]</
moodlogic_mixable>[% END %]
 
Index: server/HTML/xml/_xml_song.html
===============================================================
====
--- server/HTML/xml/_xml_song.html      (revision 3129)
+++ server/HTML/xml/_xml_song.html      (working copy)
@@ -39,5 +39,5 @@
        [% IF itemobj.modificationTime %]<modtime>[% itemobj.modificationTime | html %]</modtime>[% 
END %]
        <song_url><![CDATA[[% itempath %]]]></song_url>
        <download_url><![CDATA[[% download %]]]></download_url>
-       [% IF itemobj.coverArt('cover', 1) %]<coverart><![CDATA[/music/[% itempath | uri %]/
cover.jpg]]></coverart>[% END %]
+       [% IF itemobj.coverArt('cover', 1) %]<coverart><![CDATA[music/[% itempath | uri %]/
cover.jpg]]></coverart>[% END %]
        [% IF mixable %]<mixable>[% mixable | html %]</mixable>[% END %]
Index: server/HTML/xml/browsedb.html
===============================================================
====
--- server/HTML/xml/browsedb.html       (revision 3129)
+++ server/HTML/xml/browsedb.html       (working copy)
@@ -11,6 +11,6 @@
        [% IF browseby %]<browseby>[% browseby %]</browseby>[% END %]
        [% IF pwd_list %]<location>[% pwd_list %]</location>[% END %]
        [% IF browselist_header %][% browselist_header %][% END %]
-       [% IF coverArt %]<coverart><![CDATA[/music/[% coverArt %]/cover.jpg]]></coverart>[% END %]
+       [% IF coverArt %]<coverart><![CDATA[music/[% coverArt %]/cover.jpg]]></coverart>[% END %]
        [% browse_list %]
 </browse>