I have been using skype on a 3GS for over a year with absolutely no problems whatsoever except the occasional hang until last week since when attempting to call any phone number or skype account the sound does not function. I haven't changed any settings and I am running version 3.01(250). The phone is working absolutely perfectly otherwise.
Please try to upgrade to the version 3.5 from the App Store. If the problem persists, please reply back.
If answer was helpful please give Kudos and if issue is resolved mark it with solution for other users. Thanks!