If Showbox and YouTube are the only apps to behave like this with audio, I'd first look at just finding the latest version of Showbox, completely uninstalling the current one, then installing that latest version. Also, are there TWO YouTube apps installed on your device (click the little circle with six dots to check). If so, do both versions behave like this with audio?
If that doesn't help, check playback in Kodi/DBMC with a video add-on, something like Exodus or any you're already comfortable using.
If it does help though, as mentioned above, sign into the Play Store, but click top left Menu button, then Your apps & games, then allow all updates to be processed (hopefully including one for YouTube).