What happens chemically when flavours 'mingle'?
There are two different processes.

Amino and glutamine acids break down into smaller components, γ-glutamylpeptides due to long cooking. These proteins create the "complete, rounded taste", kokumi, similar to the umami taste, which is more a feeling than a flavor.

The second process are enzymes breaking down fat into other components that we recognize as flavor, f.e. pentylfuran or heptenal. This process takes time and is independent from temperature and happens f.e. in the fridge overnight. It is similar to the process of aging meat.

Flavor pairing is a different process that is not related to this.

Reference:
Aroma - Die Kunst des Würzens / Flavor - The art of seasoning
http://www.amazon.com/Aroma-Thomas-Vilgis-Vierich/dp/3868510729