Rosegarden's audio mixer window, reached through ->, resembles a hardware audio mixer and provides a way to see and change the state of a number of audio instruments at once. It contains many of the same controls as found in the audio and synth plugin instrument parameter boxes, most importantly a playback level meter and fader control for each audio and synth plugin instrument, as well as routing controls, pan, mono/stereo and effects plugin slots.
The audio mixer window is also where you see and control the master audio output level (at the far right of the picture above), as well as the record level (to the left of the master fader) and submaster output levels.
The audio mixer's menu is particularly relevant. Apart from controlling what will be shown in the mixer window, this menu also contains the audio routing settings for the number of stereo audio record inputs available and the number of submasters. These are studio settings whose values are saved with your Rosegarden format file.
Note that if you are not actually using any submasters (i.e. if there are no instruments configured to route their output to a submaster) you may see a small performance improvement if you set the number of submasters explicitly to None, rather than keeping unused ones in the configuration.