you can get OEM firmware
here if you reload your OS you will lose your JB and unlock so make sure you know how to redo both. Do you use a Mac or Windows? if you have a Mac or know a friend who has a Mac another choice is to get them to make you a custom firmware with the 3.1.3 that you get from iclarified then you can shift+restore to reload your phone with that and it will be reloaded already jailbroken. Then you can unlock and test your sound to see if it works.
Your phone is Jailbroken are you running any themes? perhaps the theme is either broken or doesn't have sound. If you are try a different theme or remove the theme altogether and see if it starts to work. Are you using custom ringtones? try removing them by uncheck the sync ringtones then reboot and see if you can hear them again after that you can sync them back.