Regarding the phrase in particular, it generally states:

"The film is in the post production phase," which could include audio post production, subtitling, making edits, cutting scenes, color correction etc etc etc.

Geluidsnabewerking deals in particular with the sound design and sound editing part of the audio post production phase: http://en.wikipedia.org/wiki/Audio_post_production

This could also include "mastering", although mastering is farther down the chain (i.e. the very last step in the sound editing process). In other words "afwerking van een reeds nabewerkte opname".