After a fresh install of Android Studio over Linux distributions like Fedora and Ubuntu, I experimented a very strange behavior: no sound, no audio, coming from the built-in Android emulator of Android Studio.
After many research and many attempts, I was lost and I was thinking to install Genymotion emulator, but I discovered (argh!!!) that the solution was really really simple. You need only to enter in GNOME Settings (or the same on your favourite Linux DE) and looking for the Sound (or Audio) settings. After that, search the Applications Tab (or similar).
Inside the Applications Tab, turn on (why was it off???) the qemu-system-i386 button and move the slider at the right volume setting. Go back to Android Emulator and try some audio outputs. All should be fine.