I am creating my first skill for Alexa following a Youtube channel "Alexa Developers" of the course "Building Alexa Skills from Scratch". However, after reaching the test level, I find that the skill responds with answers on screen but there is no sound at all.
In order to implement sounds, you should try using SSML features in your custom alexa skills. https://developer.amazon.com/docs/custom-skills/speech-synthesis-markup-language-ssml-reference.html