There is no safe setting that makes an iPhone speaker exceed its designed maximum output. You can, however, check the settings and conditions that commonly make audio seem too quiet.
- Press the volume-up button while the audio you care about is actively playing.
- Check the app for its own volume control.
- Remove anything covering the speaker openings and clean the exterior gently.
- Review headphone-safety or sound-limit settings if the issue occurs through headphones rather than the built-in speaker.
If the speaker remains unusually quiet or distorted at full volume, test several apps and consider hardware service instead of using apps that promise to overdrive the speaker.