Intermittent - no sound when match begins

Currently running latest client 6.3 under Windows 7 natively on iMac (Boot Camp). The problem has also happened on previous client versions back to v5.x. The sound is working in League until match begins (no sound during game play, though other applications have no issue with sound). Once Match is completed sound returns to League client and next games work fine. Problem is intermittent, but 1 to 3 in 10 games the problem occurs. During match load, I may be using Chrome browser but not playing any sounds - reading forums mostly. So it is not clear what causes the problem.