I feel foolish ... after having tried everything, but really everything conceivable but could not use WhatsApp, Viber etc. when using the Speaker phone. Normal phone calls were ok.
I finally looked for the small mike hole on top - after all, many of you recommend to cover it with a small piece of plastic - and then found that I had not properly removed the protective (almost invisible) plastic cover that the supplier had placed there to protect before shipment...
I removed it and now have a S4 the way Samsung designed it. Works perfectly. For the first time in a long time!