If you want to disable caching then configuring "http client cache memory pool 0" will do it. However, it's not recommended as every prompt played will be retrieved from the media server and this will have a significant impact on gateway call handling capacity.

The most effective way to implement caching is by configuring the Cache-Control header max-age value on prompt fetches. If you enable HTTP caching on the gateway and implement the Cache-Control header on media file retrievals then attempts to refresh the cached copy will only happen when the prompt is next used after the configured max-age value has expired. If the server copy is not newer than the cached copy then the file will not be retrieved and 304 Not Modified response is returned to the gateway HTTP client. So, enabling caching even with a very short max-age value will have much less impact on the gateway than disabling caching altogether as even though refresh attempts will be made these will only result in prompt retrieval if the server copy has been updated. If you're using the CVP VoiceXML server as the media server then there's a document here http://developer.cisco.com/web/cvp/docs that describes how to insert the Cache-Control header when using Tomcat.