Adding silence to background audio

I need to add 1.5 seconds of audio to my background audio so my first slide is silent. I know how to add it in the editor, but as soon as I "save" and close, the audio goes right back to the very begining, no 1.5 second silence.

It seems to work 1 out of every 6 times I save, and for only one project preview.

What am I doing wrong?

Sabrina