Hi @Inaí, welcome to the Box Community!
Currently, the ability to adjust audio playback speed is only available on Box.com (web app) and isn’t yet supported in the Box mobile apps.
Our Product team is always exploring ways to enhance the Box experience based on user feedback. If you'd like to suggest this feature or support similar ideas, feel free to visit Box Pulse where you can submit new requests or vote on existing ones like enhances audio playback controls.
If you have any other questions or need help with anything else, please feel free to reach out. 
You’re not missing anything — at the moment, the Box Mobile app for iPad doesn’t include playback speed controls for audio or video files, which is why you only see that option available on the web. You’re definitely not alone in wishing for it; many users find variable playback speed useful for productivity. The best step would be to share this as a feature request directly with Box through their support team or feedback channels, since that’s how they prioritize updates. In the meantime, if adjusting playback speed is critical, you could open the files in another app that supports variable playback until Box adds the option. It’s a great suggestion, and passing it along to Box is the way to get it on their radar.