This BGM is from Mouna Raagam. We can call it as 'Mohan Theme'. I found that this theme is used in many variations and in many places than the comparatively more popular Karthik theme. I like hearing to theme pieces as a single track which contains all variations of it. This is a sample of Raaja's genius in giving apt varations to a theme piece to make it fit in scenes of different moods.
Here is the next one.