I was testing out the module I wrote to support muting avatars and objects. Muting avatars and objects is working. When I right click an object or avatar, select More twice, the pie menu option Unmute them is always grayed out.
To work around this problem you have to mute another avatar or object to make the Muted Residents & Objects dialog box to appear at which point you can select entries from the list and use the Unmute button located at the bottom of the dialog box.
Singularity Viewer (64 bit) 1.8.7 (6966) running in Linux Mint 18.2