You will need to learn more about the chat cookies Yahoo puts out. They don't always put out properly formated chat cookies. As time has gone on this happens even more so as less man hours are being used in maintenance. Only the Chat cookies starting with capital letters will allow voice in chat.
I would suggest you go to YahElite -[ http://yahelite.our4m.com ]- and Irish Gaming -[ http://Irish-Gaming.Net ]- with these people you will learn more about the actual working of chat in specifics. The YahElite Chat Client running "debug mode" and "Detailed Info" will provide you with the info occurring during that specific time you want access. With the use of this info you can use Demont's SharpeVox to input the proper info gathered from YahElite's tools mentioned above. Watch Dermont's Help Video's on SharpeVox's Use.
I also note to you deleting the Temporary Internet Folders contents manually becomes necessary this along with the Content.IE5 contained within the Temporary Internet Folder that you can't see till navigated into it. This by manually typing \Content.IE5 This will contain a series of randomly named folders. Delete whats deletable within these, and do not delete the desktop.ini files. Note that if you have gone to Vista or Win7, they still use these folders. But they are no longer found in the same positions of in Documents and Settings. They are now in these more recent versions within users/local.