First, you want to make sure the volume on the Echo Dot is turned up. If it’s not, use the + and - buttons to control the volume. If it still does not work, make sure no 3.5mm AUX cable is plugged into the Dot (if it is, it will be on the back. Unplug, or make sure the speaker it’s connected to is on and the volume is up). If things still aren’t working, make sure your Echo Dot is not connected to a Bluetooth speaker. You can do this in the Alexa app, or by saying “disconnect my speaker” to your Echo Dot.
Hope this helped!