i know i can do it manually, but this was a 'tab' like 'do not disturb'. very strange indeed - seems to simply have vanished, thankfully leaving my settings behind..
as it is it's working perfectly, ie. between 10.30pm > 7.30am the only sound that works is the alarm, post / prior everything is back to 'full' volume automatically, i don't need to change any levels or press any buttons.
i know that this is apparently a feature in the new sense 5.5, or so i've read. unfortunately i live in rural australia and this xl is a 'blue tic' phone (supposedly better reception than average). i have a friend with the new htc one and he doesn't get a signal here. mind you, there's few mobiles that do ;-(