Artificial crowd noise

Artificial crowd noise is pre-recorded audio that simulates the live sounds of spectators, particularly during sporting events. 
Regular use of such audio grew during the COVID-19 pandemic when teams had to play at sporting events without spectators. The idea was to preserve a degree of normalcy for an event's viewers. Event producers mixed pre-recorded audio manually or automatically with game action. 
People have also use artificial crowd noise to simulate stadium sounds during practice, and some have accused teams of using artificial crowd noise on top of in-person crowds to distract opposing teams. 
Uses.
Some American football teams, particularly within the NFL, have used white noise or artificial crowd noise during practices to acclimate players to stadium conditions of actual games. It may also be mixed with music played at a similar volume to make it harder to tune out the increased sound.
There have been accusations on several occasions, including the Indianapolis Colts in 2007, and the Minnesota Twins during the 1987 World Series (as speculated by sportscaster Al Michaels in a 2015 interview) that teams have intentionally sweetened their in-person attendance with artificial crowd noise in an effort to distract the visiting team (with the Colts, in particular, accused of doing so to make it harder for the New England Patriots to call their plays). In the case of the former, the NFL exonerated the Colts and ruled that this had not actually occurred.
In May 2013, Swiss broadcaster SRF added artificial crowd noise to highlights of a Zürich Derby football match, where fans of both clubs had protested against heightened security measures by not entering the stadium until 10 minutes after kickoff. SRF apologized for the manipulated footage.
Events being played behind closed doors.
The COVID-19 pandemic has prompted teams to play many sporting events behind closed doors with no spectators to maintain player safety and reduce large gatherings that can spread Coronavirus disease 2019 (COVID-19).
Artificial crowd noise has often been used to preserve a degree of normalcy (via a suspension of disbelief), especially during events where a lack of crowd may be considered unusual to viewers and/or players. Audio used for this purpose is often compiled from stock of previous games (in some cases, originally compiled for use in a sports video game). The crowd noise may also include audio associated with the sport or home team, such as specific chants, and in the case of football in South Africa, the playing of vuvuzelas.
There have been varying approaches to the practice, including whether the sounds are played over a venue's audio systems or only for television viewers, and whether the audio is synchronized in real-time to correspond with in-game events. It is usually mixed by a audio engineer on-site, but some events have also employed mobile apps that allow viewers to influence the sounds by voting on reactions. For the 2020 US Open, IBM trained its Watson artificial intelligence system to cue crowd noise automatically, using footage from past editions of the tournament to determine appropriate reactions to in-game events. In some cases, artificial crowd noise has been paired with the use of augmented reality to fill in empty stands with CGI "spectators".
The practice has been met with mixed reception from viewers and sportswriters, usually dependent on the quality of the execution. One football writer argued that artificial crowds were "disingenuous" and created a disconnect with accounts of games noting the lack of spectators, and felt that viewers were missing out on the ability to hear on-field communications between players — arguing that "if a game that generally produces one of the most fierce atmospheres in the world is being played in front of a silent backdrop, that's as much a part of the story as the result."
All game shows, network & syndication, also use artificial crowd noise, especially "Price Is Right".