Make sure every piece of text in your game has a voiceover. If you don't include audio, you exclude blind players or people with limited vision from understanding your game's story or playing the game altogether.
If recording audio seems too daunting a task, consider adding screen reader compatibility. People with low vision or blindness commonly use screen readers for every day tasks like reading websites and text messages. Screen readers read text out loud using synthesized speech. For Windows, JAWS is the most common screenreader, and it has an extensive scripting language that can be included in your code. Apple's iOS has screen-reading built in too. The Xbox One has a built in narrator, as does the Playstation 4, but they are most often used to navigate menus and not for actual gameplay.
Always Include Subtitles
Make sure every piece of spoken dialog in your game has captions or subtitles. If you don't include text captions, you exclude deaf or hard-of-hearing players from understanding your game's story or playing the game altogether. Also consider people who are playing without headphones or in a noisy environment. Many hearing gamers prefer subtitles too, because they don't want to miss anything in the story.
Ideally, you should include the ability to customize caption size in your game's options menu. If this is too cumbersome, make sure you're designing your captions correctly in the first place. Use a readable typeface, like a plain sans serif font, because readers need to be able to quickly read what you have to say. Place your captions within the "safe area" of the interface, not too close to the bottom edge. White text is recommended, but you can use other light colors to denote different speakers. Add a drop shadow or shaded box around text to make it stand out from the background. Subtitles should be two lines or fewer to increase readability. Make sure the line breaks are logical, meaning they don't drop to the next line in the middle of a word or phrase. And finally, the speed shouldn't exceed 160-180 words per minute, but you can also allow players to customize the speed themselves. More on subtitles here and here.