This is a Real Annoying bug which is very hard to find, and Is actually present on LIVE ATM too.
When you play any Other gamemode/map which is not SR classic,
your **background music Dissapears**.
As we know ARAM/Howling Abyss have a BGM, which is similar to this one:
And that song completely dissapears from the game cause this bug.
Then...the bug also removes TT and Dominion Music too.
BTW the reason this bug is usually unnoticed is because "Ambiental Sound" tends to cover BGM since both tend to act together, while also several people play without music.
The Reason I post this here, is because I tested it some Days ago,
and the Bug wasnt present on My PBE files but it existed on my LOL LIVE files,
so this means the bug was recreated on PBE;
IMO the way to activate it comes from Playing Custom Games
(without other players and having bots),
and probably is made if you do not end a game properly, ending it without losing or winning the game.
Then Repairing the Client DO NOT FIX THE BUG.