Fixed retainers sit tightly against the teeth and don’t change your speech. Aligner-type retainers cover the inner surface of the teeth, so the tongue touches the retainer material when you talk. There is a slight change in how some sounds are pronounced, but you adapt quickly – within 1–2 days your tongue ‘learns’ to speak with the retainers in. With appliances that resemble a child’s removable plate, the inner part of the appliance is made of a thick layer of plastic, and this affects speech considerably. You can adapt, but it takes longer (possibly more than a month).
This answer is for general information. Your own case needs an examination by an orthodontist.