I have recently had exactly the same issue. A post on the General FS2Crew forum alerted me to a fellow who had a Realtek audio card (which I do) with a similar issue. He used the Realtek Audio Manager (via the Windows Control Panel) to set his configuration from 5.1 to stereo. I did this, exited, reboooted, went back into the Realtek Audio manager and set 5.1 again. Now it all works OK again and I can hear the FO, FA, ground crew etc. Happy days!
Hope this helps others.