I'm here late, was looking for something else and stumbled upon this question. So, in case people still do not know and up until the time yahoo changes their protocol again, if you cannot get voice in XP or Vista, you need to DL and run YCabby2, a program that contains the necessary dll files for yahoo's cam and voice functions for messenger and chat.
Search for YCabby2.zip and you will find a 2share address, where the files are presently stored. If you want the direct link, at the time, http://www.2shared.com/file/3365679/161a0057/YCabby2.html works. If yahoo changes protocol again, it will probably be YCabby3.zip that you will need to DL, close your chat client or messenger, scan the file for viruses, you should always* do this, just to be safe and then unpack the zip file, should automatically unpack to "://...WINDOWS/System32. after that, start your chat client, if using Yahelite type /renameyahvox into the text box and hit enter/return. Show YahElite where the Yahvox.exe file is in System32 and all should be working. Don't panic, it may require a restart of your chat client after "renaming" Yahvox, been a while and I don't recall.
AMF