Update for patch 26.0
- [Updated carry-over issue]: We are aware that sound from other apps can’t be played while Hearthstone is being played on some devices. This was an intentional change that was made to avoid crashes on mobile. With Patch 26.0, we’ve been able to scale back the affected devices so that now iOS is able to play sound from other apps. The fix to the underlying issue, and with it the resolution for other mobile devices, is still scheduled for a later patch.
On Android, while you have the Hearthstone app open you can swipe down from the top and unpause music that way as a work around.