Try to restart your phone and check again if it's working. Either play music or make a phone call and put it on loudspeaker. If speaker still doesn't work you should contact Doro to find out where you can bring your phone to be checked. They should be able to tell if it was a defect or broken.
Defects will be covered by warranty. Your phone will be fixed free of charge. While if they find it's broken, you will be shouldering the cost of repair.