Flash 9 uses ALSA for sound, specifically the 32-bit sound drivers. On AMD64 arch the 32-bit sound libraries (app-emulation/emul-linux-x86-soundlibs) do not seem to be dependency of flash 9. Here's a list of libraries that flash 9 requires: http://blogs.adobe.com/penguin.swf/2006/09/librarian.html
Already fixed in CVS 23 Oct 2006; Patrick McLean <chutzpah@gentoo.org> netscape-flash-9.0.21.55.ebuild: Add dep on app-emulation/emul-linux-x86-soundlibs for amd64. please sync and retry, re-open if problems persist.