I have the onn wireless keyboard model#ONA11H0087 and it's hooked to my IMAC and i am wondering if there is a way to control the volume with this keyboard like the one that came with my puter.

SOURCE: cannot control audio volume

You Have to Load the drivers for the keyboard

SOURCE: I have a wireless keyboard/mouse///model5219crf -- volume control not working

your keyboard is proably a defect

SOURCE: MS wireless laser keyboard 5000

Sorry for the delay in replying but i THINK have have the solution which I got of the Saitek forums.
I have had the same problem with VOLUME controls on two MS keyboards and a logitech, this worked a treat.

So what do you need to do?
FIRSTLY check Control Panel->Administative tools->Component services and click "Services (local)", on the RHS look for "HID input service" under STATUS it should say STARTED, if it does im not 100% if this will help but try anyway. If it doesnt, this should work.

Go to START->RUN and type REGEDIT and navigate your way to the following:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HidServ\Parameters

If there is a file called "ServiceDll" delete it (make a copy obviously and stick it on your desktop or something). Right click in the RHS of the page and create a NEW->Expandable String Value.
Call it "ServiceDll" (exactly as typed here without quotes), and make the value read "%SystemRoot%\System32\hidserv.dll" (without quotes). Now exit out of Reg edit and go back to Control Panel->Administative tools->Component services. Next to HID input service right click in the gap under STATUS and press START. Close properly and your volume should now work.
Good luck.

SOURCE: MS wireless laser keyboard 5000

Go to the Hkey_local_machine\system\currentcontrolset\services\hidserv\parameters, ServiceDll parameter defines the path to the file hidserv.dll that is needed to make sound+\- key workable. I searched for that file on my computer and found it at C:\WINDOWS\system32\drivers\ , but ServiceDll registry parameter pointed at %SystemRoot%\System32\hidserv.dll, so I changed it's value to %SystemRoot%\System32\drivers\hidserv.dll, rebooted my computer and solved the problem;)

SOURCE: volume control buttons on microsoft wireless model

reinstall the drivers for that keyboard the default windows driver are good but cant do it all. RIght click on my computer left click on proberties left click on hardware tab LC on device manager lc on keyboard double click on the keyboard device and up date your drivers.