My experience with the cockpit sounds was that they were broken with the the 2.4 update.  The only way I could get them back was to get the sound gauge from the 2.3 version and use it with the 2.4 install.  Now I can get sound by using the cfg edit and reloading the aircraft as described elsewhere in the forums.  Using the 707 gauge wasn't workable for me.  All those error messages and the sound played at the wrong volume.