I'm sure this seems to have come out of nowhere but if I have a midi track inserted and move the physical volume control slider on my midi keyboard, the [volume=101] in the track display stays at 101 even though the volume of the sound is changing to match what I'm doing with the keyboard volume control. I'm sure the number displayed used to change to match what I was hearing. Is there a setting somewhere I might have inadvertently changed that would prevent the numerical display from changing?

[volume=101]

That is not the volume fader, that is the velocity fader my friend
 
Cj

I know it's controlling velocity but it's still labelled [Volume = 127] in the track view. I'm just wondering why it no longer responds visually as a number to what I'm doing with my "Data entry" slider on my physical MIDI keyboard. I'm sure it used to so I'm just trying to figure out if I've clicked a setting somewhere without realising.

You were right the first time. It is MIDI Volume (CC7), not velocity. In order to control MIDI volume on a track in SONAR from hardware, you either need to set up your keyboard as a Control Surface (see the Reference Guide for details), or enable Remote Control of the individual MIDI track Volume widget by right-clicking it, choosing Remote Control, and setting it to respond to CC7.
 
Also, if the 101 has parens around it, then the control is disabled (the default state), and you'll need to move it once via the interface to enable it (or right-click and uncheck Disable Control).