If so, then sound in connection with flashbacks induced by a character's recalling a past event would also be considered "nondiegetic-onscreen." Obviously, this is easier to grasp--and more useful--in shorter segments (not, for example, in Million Dollar Baby, where the entire film is a flashback).
In the earlier sequences of Amadeus (1984), short flashbacks are used several times. In almost all cases, the effect of memory, the nondiegetic as the past, is enhanced by sound advances and sound lags. At 00:13:00, for example, the elderly Salieri is talking to the priest (offscreen). At 00:13:14, we hear the sound of a small fortepiano-spinet, then shortly after we see the Austrian Emperor playing with a more youthful Salieri at his side, coaching him. At the end of this short flashback, cut back to the elderly Salieri talking but the music persists for another 2-3 seconds, a sound lag.
The same device is used for another flashback shortly after (beginning at 00:14:00), and also elsewhere in the film.