When you connect your computer to the Zoom Room either via Share Screen in the Zoom app or using the HDMI cable, any sound will automatically play through the Zoom Room's speakers (behind the touchscreen). To change the volume, you can use your own computer's volume control, and the Zoom Room's speakers will adjust accordingly.
If sound is coming through your computer speakers instead of through the Zoom Room's speakers
You will need to open your computer's audio settings and manually change the output from your computer's speakers to the Zoom Room's speakers. This has been known to be an issue on certain models of the MacBook Pro.
If sound is coming simultaneously through both your computer speakers and the Zoom Room's speakers
You are likely connected to the Zoom Room via the Join meeting function rather than directly via Share Screen. If you are unable to leave the Zoom meeting and use Share Screen instead, a workaround is to remain joined to the Zoom meeting but plug a set of headphones/earbuds into your computer. This allows sound to continue coming from the Zoom Room's speakers while dampening the sound from your computer speakers.