Creating ambiance means creating music which fits perfectly to the type of atmosphere and mood that you're trying to create during the clip. Whether you want to create a happy or sad mood, it'd mainly down to the music to determine what emotions the audience feels. If the music is not correct, for example, if you had sad music playing whilst a happy action on screen, it could make the audience feel confused as they will not know how they're supposed to feel. Creating ambiance also enables the audience to emphasise with the main characters within the clip.
Invisible Among A Crowd from Patrick Mahoney on Vimeo.
The above clip is a good example of creating ambiance. The background music gives off a sense of worry and makes the audience assume something is going to happen. The tone and pace of the music are quite creepy, maybe foreshadowing what is about to happen.
Happy (HD) from HappyChow on Vimeo.
In contrast, the music in this clip gives off a very cheerful mood. It is made obvious how the audience are expected to feel whilst watching this clip, and without the music, the audience may of felt confused about their emotions towards the clip.
Elise - Student Short Film (by Aiko Ilagan) from Aiko Ilagan on Vimeo.
Again, in this clip, the music makes the audience feel worried at the beginning, and assuming something bad is going to happen, it then develops into a unhappy tone meaning the audience will also feel this way.
