Sorry for the delay here. A few things:
When I previously tested, I was unable to duplicate the voice announcement during disarm. IQ Remote was connected via PowerG. Once I switched the connection to Wi-Fi, I can replicate.
Qolsys indicates that by design, when system wide status changes happen (such as Arm to Disarm) it should vocalize, even when Night Time Mode is active. They indicate the lack of announcement when connected via PowerG is a bug.
Unfortunately, there is not a work around for the voice annunciation at this time.
A request to disable the vocalization during Night Time mode has been made to Qolsys. I could not say if/when this would be implemented.
Additionally, some changes and bug fixes are coming to both night time mode as well as the IQ Remote, in the upcoming firmware 4.6.0. There is not ETA for that but you can subscribe to the post below to be notified when 4.6.0 patch notes are added.