Since updating to iOS 13 Public Beta 4, I have lost all sound in Minecraft. The last Public Beta had fully-functioning sound and music, without issues. I am using an iPad Mini 4.
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We're actually already tracking this issue at MCPE-41651, so we will resolve and link this ticket as a duplicate.
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I found a quick workaround. Open Minecraft, wait for it to load in to the title screen, exit the app without closing it and go into another app. Then return to Minecraft. You should get sounds upon your return. This will have to be done until they can provide a proper fix.