Note: This does not affect most Pushover users, only developers that have integrated Pushover into their applications or created Pushover libraries.
Starting April 15, 2013, our API will require a valid application token to be passed to our /1/sounds.json (or .xml) endpoint, similar to our messages and receipts endpoints. Requests without a valid application token will be rejected after April 15. Application and Pushover library authors are encouraged to take this change into consideration as soon as possible.
As a reminder, the results of this API endpoint should be cached for a reasonable length of time, such as 24 hours. Our sounds list does not change very often and calling it every time your application needs to present the list of sounds to your users is wasteful.