I lose sound when exporting a recorded lecture on KeyNote to m4v

I have spent the most of the day for a recorded lecture on KeyNote (9.2) to save to a movie m4v.

The lecture is about 40 min. I the tried again with 10 min and it worked, also with 20 min, but at 30 min the sound disappeared.

How can I proceed to eave to m4v with a 40 min lecture?

Is the capacity of KeyNote lost after 30 minutes?