Which button is used to select internal stereo sources?

The correct choice for selecting internal stereo sources is the CTRL PGM button. This button is typically used to access or control the program output within a system that manages audio signals. By engaging this control, users can select and route the appropriate internal stereo sources to be processed or monitored, thereby allowing for streamlined audio management.

In contrast, the other options serve different functions. The CR PLAY button usually functions to play back audio from the control room; the MAIN button typically controls the overall output level or functionality of the main mix, affecting what is heard through the primary outputs; and the CUT L+R button functions to mute or disengage both left and right channels, which does not facilitate selection of audio sources but rather silence them. Understanding the role of the CTRL PGM button allows for effective management of internal audio sources in a mixing or audio production environment.