Best answer by Keith N
If you use other apps that use the iOS device audio, such as making a phone call, YouTube, social media apps, or use Bluetooth headphones while playing from “This iPhone or iPad,” you may experience audio interruptions on Sonos until you completely close or quit those apps. Until this issue is resolved, we also recommend disabling Bluetooth for the best experience.
Additionally, if there are any AirPlay devices on your network, such as an Apple TV or Airport Express, audio from “This iPhone or iPad” may stop abruptly until AirPlay is disabled on those devices.
We recognize this isn't an ideal solution for those who use AirPlay regularly at home, but if you prefer to continue playing to Sonos from “This iPhone or iPad” without interruption, refer to the device manufacturer’s instructions for disabling AirPlay. Once AirPlay is disabled, close and re-open the Sonos app and start playback again.