Don't worry, the sound options aren't gone. We simply changed where you can find them.
Before 3.4, some players sometimes experienced issues where changing the sound in Vainglory would affect some other apps' sound settings. Now, you can enjoy your favourite music application while playing Vainglory, unless you already love the music in-game (who wouldn't?).
To find your sound options (as well as some other settings), start a match and simply tap on the cogwheel located on the top right side of your screen:
Now you can dance your heart away to your favourite tune every time you shatter your enemy's crystal, enjoy!