Do you have BSD and SSH installed? If you installed those, then at some point did a hard reboot, that might be the reason you lost your sound. Same thing happened to me after jailbreaking 1.1.3.
If so, try going to "settings" then "general" then "reset" then "reset all settings", then after that, do a hard reboot on your iPhone. If that was the cause, your sounds will return and will no longer be a problem in the future, even if you happen to do another hard reboot.
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