You can silent certain apps when launched by using the app listing feature available in this tweak. Using this feature you can also select a custom value for volume in order to hear your music at the right volume when the app is launched. You can use MyVolume Tweak with Activator. So for a wide variety of volume related controls now you can specify gestures. MyVolume tweak is compatible with iPhone, iPod touch and iPad running iOS version 3.2 or later.
MyVolume is available in Cydia. It costs $1.99 USD. You will need to have a jailbroken iPhone, iPod touch and iPad in order to install this tweak. If you don’t have jailbroken iPhone, iPod touch and iPad then you can follow the step by step instructions posted here to jailbreak your iPhone, iPod touch and iPad on iOS 4.2.1 with Redsn0w.