Not all DE's, for example GNOME, support compressed SVG icons for desktop entries, and the XDG icon theme specification does not mention svgz at all.
http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html
I suggest installing the kid3 and kid3-qt SVG icons as uncompressed svg instead of svgz.
I have changed the build system to use an uncompressed SVG icon for kid3-qt. For the KDE package kid3, there is still an SVGZ icon, because the KDE build system only supports svgz, see the KDE4_INSTALL_ICONS macro in KDE4Macros.cmake.
Fixed in version 2.1.