Can I export/save as an impress file to MP4 video keeping timings
I have several videos (in PPTX format) of hymns with word overlays and wish to save to a video format pref MP4 but preserving the timings. Is this possible in Libreoffice?
Sorry, I think you're confusing some terms.
There are no videos in PPTX format.
A file in PPTX format is a slide show.
In LibreOffice you can open a PPTX file. You can test if the time schedule is respected by downloading and installing LibreOffice.
LibreOffice uses the ODP format for screen presentations. There is no 100% compatibility between these file formats (PPTX and ODP).
If you want to pack a screen presentation into a MP4 (video) format, you need a screencast program.
To clarify/supplement what exactly you want to achieve, please edit your initial question.
For long term LO users - apparently there is a function in Microsoft Powerpoint which exports a powerpoint presentation to a video forms (mp4). This is used to burn to a DVD or live stream it with some services and avoid the requirement for software required to view powerpoint slides and hence only rely on the ability to decode an mp4 / video file.