These remotes have a feature called Volume Punch Through. Most cable boxes have no useful volume function, and the remote is setup by default to make the Vol+, Vol-, and Mute buttons actually control the TV device. You probably can notice the TV button lighting up when you press one of those buttons while in CBL mode.

Change the VPT to point at the AUX device.

Press Setup until 2 blinks
Enter 993 (should get 2 blinks)
Press AUX (AUX should blink twice)

Now pressing Vol+, Vol-, or Mute in any device mode will act as if the remote were in AUX mode.

This programming is described in the remote's instruction sheet, downloadable from www.urcsupport.com/index.php.

BTW, trying to assign those EFCs to the cable button wouldn't have worked even if VPT were disabled, because the EFCs are chosen to work with the 31758 code rather than the 01376 setup code on the CBL button.  You would need to first temporarily setup 31758 on the cable button, do the 994 EFC assignment, and then restore the 01376 setup code.  And to change the CBL setup code, the Cable Lock feature would need to be disabled using 982.    (Comcast doesn't want you to accidentally change that CBL setup code.)