This stuff does just work in MC. At a guess, Foobar may have been capturing the media key commands. Maybe.
If you still have a machine with a working trial, import some music into MC, kill everything Foobar, and try using the media keys to control volume with just MC running. It should work.
Also, if this was the only major issue you found with MC, buy it and then keep working on this issue to sort it out. Or just change over completely to MC as your player. A search of the forum will also find other threads where people discuss using Foobar with MC.
Yeah I'm going to buy it when the trial ends. But still would like to get a fix for this problem.
So upon deeper inspection, this problem may be troublesome. The volume for "Speaker" (main output of my Asus Xonax U7) affects the JRiver volume. But of course, since WDM is on default for sound output, the volume keys change the WDM driver's volume, which does nothing. However, even when WDM is on default, changing the volume for "Speaker" under Control Panel --> Sound will change the volume.
I attempted to use Nircmd to change the "Speaker" output's volume even though JRiver's WDM is the output default with a single command. Then I used AutoHotKey to remap my volume control keys to run that nircmd command. That does not work because I'm not mapping the right keyboard keys. Turns out, the volume key on a MacBook under Bootcamp isn't actually a key. When I used a keylogger to see what the volume up key is, it was never actually captured even though every other non Fn key is captured. This most likely means that when F1-F12 keys are pressed, the default is Fn + F1-F12 (since other than us programmers, who uses F1-F12 keys?), Apple's keyboard driver is simply converting it to a command to do various actions like adjust brightness and volume, and therefore the keylogger never captures it.
So, ummm, maybe get a MacBook with Windows installed and try it out?