There is much more to a film's sound design than mere dialogue and soundtrack.
Sounds can be created and recorded in a variety of different ways to different ends and for different effects.
Some aspects of sound design include:
Volume (Are there extreme constrasts? Are some scenes quiet and others loud?)
Fidelity or Naturalness (Is the sound realistic or artificial? Perhaps the sound editor wants a slightly artificial sound rather than a realistic one).
Apparent Microphone Distance (Are camera and microphone distance radically different?)
Mixing (What sounds are emphasized? Are some turned up? Others turned down? Some silenced altogether?)
In most cases, decisions about any of these terms will not have a gratuitous effect. They are part of an overall design and artistic vision for the film.
Read Hayward's entries on Asynchronous/synchronous sound, music, sound/soundtrack.