Each time you select and regenerate a section you create a separate piece of audio track which will play successively with the other sections, but the joints may be heard.. Look in the Classic Tracks Window and at the last column to the right "EVENTS". For an Audio Track this number should be ONE unless CC7 or CC10 controller data has been included. The same information can be viewed in the Event List use Control + F2.

Use Track > Consolidate Audio Region to change the various individual audio files to one contiguous audio track to remove clicks due to multiple audio sections.