Sound mystery solved - PureRAW3 wasn’t the culprit after all. Lightroom was the noisemaker - it seems to default to making sounds on every single image import/export when interacting with PureRAW3. Configurable from its Preferences pane.
Very strange - I’ve just confirmed again. When batch processing, there’s a “ding” sound after every single image completion. And there PureRAW does not show up in the list of apps for which Notifications are enabled, so I have no way of turning off its sounds via the System Settings.