E3 Air Joy Pro

Is 7.1 surround sound applicable to mobile phone?

No, 7.1 Surround sound can only work on PC with USB cable plugged. 

The volume button always worked fine, but suddenly turn to disabled.

Please check whether you have mistakenly muted your device somewhere.

The microphone had a sort of tinny sound to it and was not picking up my voice more than the ambient noise around it. 

Please try to test the microphone on other devices to see whether it can normally record the sound.

Sound is low and it's not easy to differentiate between foot steps and gunshots.

 It might result from distorted sound. Kindly check whether your headset has been well connected with your device. But please note that the sound of gaming headsets are usually lower.