Same problem here got this from Razer site:
http://help.razersupport.com/us-en/resolution.asp?sid=222715090724118090126000&pid=400000001&pnm=Razer+Megalodon&seid=400000002&pos=N%2FA&top=Audio&rid=400001172
I have done all of it but mine still drops sound after awhile, problem is not restricted to ARMA2 but also happens in other games.
The only way I see this getting fixed is by Razer with a Firmware update or Microsoft with a driver update.
Edit: Temp Fix: Try Changing the Windows Speaker config to 5.1
Right Click
Select 5.1
Keep hitting next but make sure all boxes are ticked, then Finish when done.