If that doesn't work, leave, /reloadui, join again.
For the sound join an XBL party, then switch back to game chat.
Great, that fixes the bug completely and isn't at all a workaround.
I don't like XBL parties and prefer game chat. I also prefer not having chat channels disappearing, which is probably a tall order, but acknowledging the bug within a year is something ZOS could do pretty easily.
It is disappointing that this has been around for over a year and hasn't been acknowledged. I'm sure in-game chat is not easy code to debug, but it would help if someone at ZOS (@ZOS_GinaBruno, @ZOS_KaiSchober, @ZOS_JessicaFolsom, @ZOS_BillE or @ZOS_GaryA) would at least let us know they are aware of the bug. Maybe let us know if there is a way we can help get some data for debugging the issue.