i have figured out wot causes this! (ill take the gold medal plz)
when u press \ and go to options, u can alter the volume of the in-game sounds and music, if u turn the sound right down, obviously it goes quiet (duh) but when u turn it back up again and dont relog some sounds stay quiet and some go back to normal.
i mebbe realise this mebbe isnt ur issues, but mebbe check it out and mebbe tell me that i mebbe right!