Hello,
Can I ask why your web service is taking several seconds to respond? Could you not speed this up?
As far as I'm aware, telling mod_curl to play music when a request is being executed is really not the right way to look at it. If you need to retrieve data from an external service that will take several seconds, you would place the call into hold or into a parking queue, execute the request, then come back.. however you'd do this within the mod_curl response, rather than it happening on a mod_curl call itself. For example, you could get the mod_curl to response immediately, then use a mixture of httpapi/includes/parking to achieve your goal..
Perhaps someone else might be able to offer a cleaner solution, but that's the best I can think of at the moment.
Hope this helps!
Cal