In the default setting, the app uses preview images for the preview playback. This will keep the audio and video playback in sync. Especially on devices with slower hardware.
You can change the preview quality in the app settings. Please note a high preview quality setting may result in frame drops. These frame drops happen automatically, otherwise, your audio would no longer be in sync.
Rest assured, when exporting the video, the app will use high-quality images only, regardless of the preview settings.