On my system libcanberra was installed without GTK2 useflag. aseqview emerges without error, however when starting it outputs something like:
Gtk-Message: 14:54:32.742: Failed to load module "canberra-gtk-module"
and then quits.
i added USE=gtk2 to libcanberra and that made it work.
(as a sidenote, i had to run aseqview with -P, which disables the piano view. else it would crash - that could be an issue with how it uses GTK, and is certainly another bug :))