Volume is already set to shift+select and shift+start for the launcher and by default in retroarch. If you mean in ALL applications, take a look at this thread Control backlight brightness everywhere! it shouldn’t be too hard to create something similar to handle volume, just be aware, that there are limited keys on the gameshell so this type of scripts can be intrusive to anything that needs to use whatever key combo is being used.
If you are referring to an issue with the volume not working with shift+select / shift+start in the launcher or retroarch, please describe you problem in more detail, and anything you may have done recently to cause the issue.