EDIT - Neil, nope,, i loaded my track (Spitfire vs 3 HE111) just a little track but works fine, both focusing on any single plane and/or switching to panoramic flyby views (F3) that show the HE111's chased by my Spit... I can hear all the engines.
Interesting, good to hear it's working ok for you. System specs might hold some clues, possibly ingame settings. Sound settings in conf.ini might be usefull too.