a real world point of view
A dubbing mixer is a member of a post-production team and is responsible for editing together the dialog, music and sound effects to create the final soundtrack for a production. Dubbing mixers ensure the sound in a film or television program is technically correct and as near to the director's or original idea as possible.
The dubbing mixer achieves this by performing tasks such as mixing and cross-fading the sound, using the soundtrack prepared by the music editor.
In practice, the dubbing mixer is the head of the post-production division of the sound department. In the BBC Wales era, this title has almost exclusively been held by Tim Ricketts, but a few other people have very occasionally substituted for him.