hmm no expert but it does sound like it is relating to you using a shell for kodi, like kodi doesn't minimize properly to allow bigbox control, which results in no sound.....just a guess though. Going back to advanced launcher, i think i didnt explain myself well there lol, happens a lot :), you can use adv laucnher to point to ur install of bigbox and it will launch it identical to the big box app, but with the benefit that adv launcher has a few more options that may solve your problem.
So, if it was me i would grab adv launcher, point it to load bigbox, and play with some options, if that works, would be identical to ur current setup, but with sound :) .....again im no expert though, just thinking about what i would try if i was in ur situation.