Alsa 0.9.1 is a backwards compatible update from alsa 0.9.0. There have been many bug fixes and transparent feature enhancements. I have been using 0.9.1 intensly for a few weeks now and it shows no reasons for being masked any longer Reproducible: Always Steps to Reproduce: 1. 2. 3.
I would surgest you write your comments here: http://stable.gentoo.org/
Alsa 0.9.1 is no longer on portage. It was superceded by Alsa 0.9.2 on unstable quite some time back. There are some fundamental differences between 0.9.2 and the stable version 0.9.0 like the non-use of libasound.so.1 which I suppose is being tested. For what it's worth I've been using 0.9.2 and everything seems fine.
Now that the portage tree was unfrozen, I guess it would be safe to unmask 0.9.2 packages. I have just marked the following packages stable on x86: media-sound/alsa-driver-0.9.2 media-libs/alsa-lib-0.9.2 media-sound/alsa-tools-0.9.1 media-sound/alsa-utils-0.9.2 media-libs/alsa-oss-0.9.1 Please test. Thanks.