Unfortunately Pidgin does not support voice/video for Yahoo! Messenger. This feature has been requested years ago but has still not been implemented.

Another application, GYachI, does have this functionality, so you might consider using that if you need to use Yahoo!'s instant messaging service for voice and video chat. Or you could use a different protocol/service for this that is supported in Pidgin.

Unfortunately, it doesn't look like GYachI has been maintained much recently. So you may or may not get it to work either. If you're running Ubuntu 10.04 LTS, you can install it with this PPA. (Read this first though, so you understand the risks of PPA's and how to judge whether or not a PPA is safe.) Otherwise, you'll have to build the program from source. To do this, download the source code by choosing the source code option on the GYachI downloads page. If you don't know how to build a program from source on Ubuntu, see this guide and/or this guide.