1. A method comprising: receiving a request for content corresponding to a content domain of a voice interface, the request comprising a voice command matching a domain vocabulary of the content domain that includes a term recognizable within the content domain, wherein the content domain represents an information group related to a common topic or subject and is located within a domain hierarchy of the voice interface including a plurality of content domains; determining whether the request satisfies a request threshold of a minimum number of requests made to the content domain; adding the term of the domain vocabulary to a global vocabulary for the voice interface based on satisfaction of the request threshold, wherein the term added to the global vocabulary is recognizable by the voice interface at any recognition instance within the domain hierarchy; and repeating the determining and the adding for additional received requests for content.