Firstly, we would like you to know that once you have purchased a product, you can download it and playback within the app even when you're offline. There is an “Export” feature on the latest version of the app (version 4.0.5 for Androids), which allows you to export downloaded files to external storage devices.
Please be assured that when you download the messages in the app, you would be able to play them when you're offline.
If you encounter any further issues, please send a mail to support@pastorchrisdigitallibrary.org, and you will be duly attended to.