Here is a very nice Cydia tweak NowNow that let you allow to replace Siri with Google Voice Search on iOS. If you install NowNow on your jailbroken iDevice, then you can activate it to get access to Google Voice Search on your iOS device, from anywhere that replaces Apple s voice assistant Siri.
Download Google Search for iOS
Now, if you want to replace Siri with Google Voice Search then first install, latest version of Google Search for iOS, and then go to BigBoss repo in Cydia to download and install NowNow, which is available for free.
After installing NowNow, first you have to restart your Springboard, then just to activate NowNow, go through “Settings app” from Springboard and navigate to “NowNow” > “Activation”, and thats all.
The last step to use Google Voice Search on iOS instead of Siri, just hold on device “Home” button for a while which will enable it through NowNow.
So, just try out Google Voice Search on iOS instead of Siri, and write down your experience in comments.