By the way it may just be a loose connection with the bottom speaker module, which although you could investigate, and maybe fix, clearly this was not what you wanted with a new phone. I say this partly as it may take a while for Fairphone to get back to you and even then they may ask you to do tests etc. which can be very annoying.
Taking the phone apart to check the speaker module is seated correctly doesn’t invalidate the warranty and may be a quicker fix if that is the problem but you had better ask for official support and repair immediately. And even if you do fix it you may feel insecure that the same or another problem may arise and be equally or even more distressing.
So once you have made the official support request look at the following link which details how to take the phone apart. You may feel nervous about doing so but reading it will show it is possible and you could do it.
All the best