It is not possible for any iOS application to play a custom, user-defined sound when receiving a push notification.

This is a limitation of iOS and we have no way around that.

When receiving a notification, it is only possible to play a sound file that is included in the application package submitted to the App Store. Thus, no user defined custom sound can be played.

That said, if you have a type of sound that you think should really be included in Boxcar, please feel free to submit a feature request.