Two companies have pleaded guilty to breaching health and safety laws after part of a stage collapsed in 2003.
Some people received broken bones in the incident
Thirty people were injured when the stage fell during a production of Sing-Along-A-Sound-of-Music at Birmingham's Alexandra Theatre.
Producers Sing Along Productions and theatre managers Clear Channel said the accident was a result of a communications breakdown between them.
The case has been sent to Birmingham Crown Court for sentencing.
Audience members in fancy dress had climbed onto the stage to join in a song when the stage partially collapsed, sending them 20ft into the orchestra pit.
Injuries ranged from cuts and bruises to multiple fractures.
The case was brought to Birmingham Magistrates Court by Birmingham City Council under health and safety legislation.