While continuing to work on an hour long video shoot in Final Cut Pro X (FCPX)
with three Canon M-500 cameras, one Sony RX-100 and a Zoom H4n recorder, we ran into the problem of needing to adjust the volume on mulitple clips simultanously.
To adjust the volume of multiple / several clips simultaneously select the clips to be adjusted with the mouse.
Then click Control + or Control -.
Doing this will increase or decrease the volume of all the selected clips by one dB (1dB) increments.