In American radio, film, television, and video games, walla is a sound effect imitating the murmur of a crowd in the background. A group of actors brought together in the post-production stage of film production to create this murmur is known as a walla group. According to one story, walla received its name during the early days of radio, when it was discovered that having several people repeat the sound walla in the background was sufficient to mimic the indistinct chatter of a crowd. Nowadays, walla actors make use of real words and conversations, often improvised, tailored to the languages, speech patterns, and accents that might be expected of the crowd to be mimicked.

Walla is called rhubarb in the UK, rhabarber in Germany, rabarber in the Netherlands and Flanders (Belgium) as well as Sweden, and gaya in Japan, perhaps in part reflecting the varying textures of crowd noise in the different countries. Other phrases are "carrots and peas," "watermelon cantaloupe, watermelon cantaloupe" and "natter natter" (to which the response is "grommish grommish").

Walla is sometimes turned into an in-joke. The TV show South Park often parodies walla by having angry mobs mutter "rabble rabble rabble," sometimes clearly and distinctly. In an episode of Harvey Birdman Attorney at Law, a distraught courtroom audience distinctly and repeatedly shouts "rutabaga", a reference to the use of the term "rhubarb". In the Steve Martin film The Man With Two Brains, the audience at a scientific presentation is quite clearly heard to be saying "murmur, murmur" after Martin invites them to "murmur all you want." French and Saunders often make use of the phrase clearly and distinctly during sketches that feature film shoot extras.

While it is generally expensive for the film makers to put distinct words in a specific background performer's mouth (as this would turn "extras" into actors during the sound mix, meaning they must be paid more), this problem can be avoided by recording gibberish that syncs with the on-screen mouth movements ("lip flap") of a specific background performer. It is thereby possible to make it sound as though an extra is saying something, when in fact they are not delivering any actual dialogue. This gibberish is known as "Snazzum", referring to the way in which the cartoon character Yosemite Sam would swear when angry ("Yassin Sassin Snazzum Frazzum!").

The British comedian Eric Sykes wrote, directed and starred in the 1969 film Rhubarb, in which all of the actors' dialogue consists of the word "rhubarb" repeated over and over. This gives the finished movie the general feeling of a silent film because it has no coherent dialogue, but with the crucial difference that the "rhubarb" dialogue still conveys the characters' emotions and moods.

In Season 5, Episode 3 ("Let's Stay Together") of 30 Rock, Rob Reiner repeats the phrase, "Rhubarb, Rhubarb, Peas and Carrots..." in a scene when a group of congressmen and women are muttering indistinctive chatter. A similar reference is made by Jack Donaghy's assistant, Jonathan, in Season 4, Episode 14 ("Future Husband").