Libratone speakers are not able to receive sound directly from Google Cast App.
If you wish to "Google Cast" enable your Libratone speaker, you must buy Chromecast Audio and connect your ChromeCast Audio to AUX input on your Libratone speaker.
Google Cast App is compatible with several iOS and Android music apps.
To play local music content from iOS we recommend AllCast App by AllCast LLC. From Android you can also use AllCast.
Read more about Chromecast for Audio and compatible apps here.