The biggest cause of voice not working is the TSP Codec which doesnt seem to be included with Windows Vista or Windows 7 so the first thing to do to fix this issue is install the TSPCodec you can download and install the TSP Codec by clicking here. Make sure you close any program that connects to Yahoo chat before installing such as Y!Epic, Yahoo! Messenger etc.

The Second issue with the TSP Codec is if you dont have a microphone or you have a built in microphone then the install of the TSP Codec will not fix voice not 100% sure why this happens but if you dont have a microphone to plug in while installing the TSP Codec then plug headphones into the mic port and then install the TSP Codec. Once TSP Codec is installed make sure voice is working and then you can remove the headphones you used to trick the installer into thinking you have a microphone.

If you still not able to connect to voice your firewall may be blocking it try disabling your firewall and see if voice works if it does you will have to configure your firewall to allow voice.