emerge alsa-plugins -vp

These are the packages that would be merged, in order:

Calculating dependencies... done!

emerge: there are no ebuilds built with USE flags to satisfy "media-libs/alsa-lib[alsa_pcm_plugins_rate,alsa_pcm_plugins_plug]".
!!! One of the following packages is required to complete your request:
- media-libs/alsa-lib-1.0.24.1 (Change USE: +alsa_pcm_plugins_plug +alsa_pcm_plugins_rate)
(dependency required by "media-plugins/alsa-plugins-1.0.24[-speex,ffmpeg,-libsamplerate]" [ebuild])
(dependency required by "alsa-plugins" [argument])