(Assuming that you're using the right version of ventrilo)

Some of the codecs that ventrilo supports are patented by other companies. For this reason, ventrilo doesn't ship with all the codecs it supports. Which codecs you need, depends on the codecs that have been enabled on the server. Most servers use the codecs that are shipped with the client's operating system (standard GSM codecs, for instance), but some use non-standard codecs (usually for reduce bandwidth usage). Have you tried installing the codecs that it asks for?