OnePlus
Change Select to Speak Voice Speed and Language auf OnePlus
Select to Speak on OnePlus phones uses the same Text to speech engine as TalkBack, but it runs as a separate accessibility tool for reading selected text aloud on demand.
If the voice sounds too fast, too slow, or speaks in a language you did not choose, the settings are hidden one level below the main Select to Speak toggle, inside the shared Text to speech settings.
Open Settings ➔ System ➔ Accessibility ➔ Select to Speak, then tap the gear icon to open Settings ➔ Text to speech settings.
From there you can set the Language, or use the sliders to adjust Speech rate or Pitch. Moving the Speech rate slider to the right increases how fast the voice reads, while moving it left slows it down.
Pitch works independently from speed, so you can keep a natural-sounding voice even at a higher reading rate.
These settings apply system-wide to the Text to speech engine, so changes here also affect other apps that use the same engine, not only Select to Speak.
If the voice still sounds off after adjusting rate and pitch, check whether a different Text to speech engine is installed and selected as default, since some third-party engines offer more voice options.