The sound pretty often mutes after spending some time outside of a game (locked screen or doing other stuff while the game's tab is active in background)
This problem appears on my Android Sony Xperia Z3 compact and I also experienced the problem on my girlfriend's Samsung Alpha and also iOS some time ago.
(Everything's on Chrome)
Looks pretty important to me . I also understand that it could be Chrome's bug, so I will report this to them
P.S. You can try to reproduce this with any C2 web exported game. Just lock the screen, wait for like 10mins and/or do some stuff in background and/or wait for your device to go to sleep and wake it up after ~10mins and the sound could be gone.
Will try to find a workaround to avoid this