When you first open the game when it's not loaded the sound will work just fine, but if you’re to minimize Minecraft to open another app and then get back on Minecraft, there is any sound whatsoever in the game. This forces you to close and reopen the game. Sorry if this is a duplicate but I’ve looked at the reports and didn’t find anything reporting this.
[media]Related issues
Comments

Thank you for your report!
We're actually already tracking this issue at MCPE-101027, so I will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki