"Auto Silent” app is the answer for all your iPhone sound issues!

You can use “Auto Silent” app to disable or bypass iPhone Silent switch. If your switch was broken in silent mode, it will restore all sounds including ring. Then you can use Auto Silent app to put phone into silent mode. There is SBSettings toggle to quickly mute and unmute the phone using Auto Silent app.
The app supports iPhone 2G, 3G and 3GS and OS firmware 2.2, 3.0, 3.1
This app can also be used by those people whose silent switch is loose or accidently put phone into silent mode.
There is provision to put phone into silent mode automatically using pre-configured settings in the app or according to appointments in iPhone Calendar.
There is Provision to put phone into silent mode based on timer. The phone will come out of silent mode automatically once timer goes off. Useful for people who puts phone into silent mode but forget to unmute it. Useful when you go for a movie and forget to unmute once movie is done.
More info at iPhone Packers: iPhone dev team
The app can be downloaded through Cydia. Search for “Auto Silent” on Cydia. You neeed to jailbreak in order to install Cydia.

Search for “How to jailbreak iPhone” on bing or google.