Audio sweetening is a "catchall" phrase for fine-tuning sound  in postproduction. 

Sweeten/sweetener refer to subtly mixing an additional sound to a pre existing sound to "sweeten" the pre-existing sound. (As in sweets, sugar.)

For example in the film "Backdraft", the sounds of wolves howling were faintly added to the sound of fire crackling to sweeten and give it a haunting, animal like quality to it.