The sound designer plans and provides the sound effects in the play, including music from existing sources. In addition, a composer may write original music for the show. All the music and/or effects in a play considered as a whole make up the "soundscape." - http://www.aact.org/sound-designer

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Note added at 1 hr (2015-06-24 00:46:48 GMT)
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What's the difference between a soundscape and well... a sound design?

A sound design is the complete set of music, sound effects, and reinforcement for a production. A soundscape is an on going environment created by sounds. Sound designs may include several small soundscapes, or they may BE one continuously running effects bed.
A soundscape's purpose is to support events of a play, helping to draw the audience into the time and place in which the show is set, with or without reinforcement of other design elements. - http://www.theatreface.com/profiles/blogs/sound-scape-ing

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The Function of the Soundscape
Historical Background
When we examine some of the historical perspectives regarding the function of sound in the theatre we discover a multitude of attempts to define that function. Harold Burris-Meyer provided the first attempt at codifying the functions of sound in the drama in his landmark text, Sound in the Theatre. He suggested eight functions for sound in the theatre:

1. To transmit the human voice in speech or song (adequate audibility is always the first requisite);
2. To establish locale (bird songs, traffic noises);
3. To establish atmosphere (wind and rain);
4. To create and sustain mood (combinations of devices used for locale and atmosphere; distortion of speech; soft music);
5. As an independent arbitrary emotional stimulus (music, non-associative sounds);
6. As an actor (the voice of the Living Newspaper);
7. To reveal character (the unspoken aside);
8. To advance the plot (sound bridges between scenes or episodes) [Burris-Meyer 2]

David Collison attempted to quantify the functions of sound effects in his book Stage Sound by suggesting five functions:

1. To establish (a) locale (b) time of year (c) day or night (d) weather conditions;
2. To evoke atmosphere;
3. To link scenes;
4. As an emotional stimulus;
5. To reproduce physical happenings: spot cues like cars arriving, babies crying, clocks striking, elephants falling out of trees, etc. [Collison 75]

Carol Waaser also followed with five functions in her book Sound and Music for the Theatre:

1. Realistic effects called for in the Script;
2. Voiceovers;
3. Punctuation effects for entrances and exits, ends of scenes;
4. Underscoring for mood or character themes;
5. Preshow atmosphere. [Waaser 67-69]

John Bracewell identified seven in his excellent text, Sound Design in the Theatre:

1. Audibility;
2. Motivation;
3. Music;
4. Vocal alteration;
5. Vocal substitution;
6. Extension of dramatic space/time;
7. Mood. [Bracewell 207]

http://goo.gl/bAMDUy - The Function of the Soundscape