Custom theme no sound
I had the same issue. Enabling custom theme with Default sound pack would result in no sound on Android. I have sent a bug report to chess.com
In the meantime, 2 workarounds:
1 - Pick "Wood" theme, and then customize as you wish (background, pieces, board). It might work with other themes (didn't work with Default theme though) but that fixed the sound issue for me (without reinstalling nor restarting whatsoever)
2 - Another option: set your custom theme as you wish, then pick a Custom theme sound such as Newspaper. Plug the phone in the your pc, and browse to chess.com app cache. for me it was in Internal storage/Android/data/com.chess/cache/sounds/. There, replace the audio files of the working theme with those of the theme that doesn't work (this will trick the app in thinking it plays the working theme but the actual files are the other). However, I think I noticed some of the files of the custom theme were actually empty (no sound) which might be what caused it in the first place.
We are working on this and think we might have a solution soon. This bug has been pestering us for a while now.