In Speech Synthesis, you can provide feedback on generated audio by clicking either thumbs up/down in the bottom right corner of the audio player. This option will appear after the generation has completed or when playing the selected file from your history.
In Studio, you can provide feedback on any audio issues in a specific paragraph by clicking the Feedback button in the toolbar with the paragraph selected. You can also leave written feedback by clicking Project Options and selecting Submit feedback. This will open a feedback form.