Released 06.07.2013. >=1.0.0 are ABI incompatible with earlier. Among other changes, the few dependent libs were merged into the main lib.
Of all of portage, ony media-sound/sidplayfp-0.3.3 and media-plugins/audacious-plugins-3.4.1 depend on media-libs/libsidplayfp. Out of those, the audacious-plugins don't even compile with the old version in portage. I would suggest to bump the player and the library.
1.1.0 in portage