You have a few difference options when wanting to run sound in mono on the X32.  

First, and simplest, you can pan all channels Left and come out of Analog Output 15.  This way, you still can use the MAIN LR fader (far right) too.  

Second option would be to use the Mono Bus (located above the "Pan" encoder within the MAIN BUS section).  For each channel, you'll need to press the SELECT button then press the MONO button, then turn up the LEVEL encoder (above MONO button) to 0dB.  Repeat for all channels.  You'll then need to assign an Analog Output (ROUTING) for the MONO Bus, this will be described as "MAIN M/C".  

The third option would be to send the MAIN LR to a Matrix.  The Matrix, say Matrix 1, would now act as the Main fader.  To do this- press the SELECT button above the MAIN LR fader.  Next, press the VIEW button within the BUS SENDS section.  Turn the first encoder below the screen to send the MAIN LR signal to Matrix 1.  This will sum both left and right to mono, too.  You will then need to assign an Analog Output (ROUTING) for Maxtrix 1.