Whilst playing the my last game of LOL for the day, I noticed that the champion select screen music was still playing.
I asked in all-chat if that was the same for anyone else, one replied saying he wouldn't know since he muted the audio, so it gave the idea to mute audio for the game, the music however was still there.
The only theory I have is that it had something to do with a few players leaving and rejoining the lobby quickly (not sure what causes that, but it must be a connection problem)
I did check the replay to see if it still happened, yet it only caught my eye to another minor bug (which I shall write a report on soon)
I shall reply to this if it does happen again.