What device type are you using? There may be physical buttons on the device that are pre-programmed for Volume control. If not, you can build a custom TE script that adjusts the system volume based on a specified key sequence.
Here are some sample scripts showing how to accomplish this for the scanner beep:
Intermec CK3R, and the older units CK3B.
They do appear to have the physical buttons on the device - The images indicate that the blue control key + 5 or 0 will move the volume up or down, but neither seem to work, I don't know if these have been overridden by the Wavelink software.
I manged to set some controls within the keyboard creator to allow user to adjust system volume