Asterisk PBX sends voicemail messages attached to a notification e-mail. The voicemail recording is encoded with ms-gsm codec which is handled by the gsm plugin from gst-plugins-bad. Unfortunately the gsm plugin is not built, because it is listed in gst-plugins-bad.eclass inside the my_gst_plugins_bad variable. A separate ebuild is not available either. After removing gsm from the above-mentioned variable and reemerging gst-plugins-bad, I was able to replay the voicemail without any problems. Please remove the gsm entry from the eclass or provide a separate ebuild for gst-plugins-gsm.
media-plugins/gst-plugins-gsm-0.10.22 added to tree, thanks. If you need keywords for other arches than the ~amd64 I was able to test, then proceed as normal for keyword request, asking them for the arch(es) you have tested and need in a new bug.