The G510s media keys should work like those on a standard keyboard when no headset is plugged in. When a headset is plugged in, the G510s restarts, and then the keyboard audio controls work with the system like the audio controls on a USB headset (see answer 360023252874).
On some operating systems, like Windows 7, headset audio controls only control the volume levels on the headset itself. The G510s audio controls will only change the volume of the device plugged into the G510s on these operating systems.
To enable the keyboard's media keys to work for all devices, simply disconnect your headset from the G510s audio port.