Keynote on iPhone6, videos has no sound!

Once playing a presentation with Keynote on my iPhone, all the videos in it are played with no sound!

This happens either with a presentation created on iphone or with one created on my desktop computer and imported on my iphone.

I tried to search the preferences, but found out nothing that helps.

Very strange and disappointing, as I'm a filmmaker and love the presentation made with keynote. But with no sound it is useless.

Any help before going to the Apple Store?

Cheers,

Andrea