Unusual Sound problem in FS2002 - Query

A friend of mine has a strange sound problem in FS2002.The sounds for the Cessna, Beech etc. are working absolutely perfectly. However, all the other Aircraft sounds(Boeings,Lear,Bell,) are at such a low volume that even when turned up fully can barely be heard.All sounds in his other apps are working fine.A fresh install with latest drivers and DX8.1 did nothing to solve.You would have imagined that, with FS sounds, either they'd ALL be working or none would be working - no in between.Has anyone got any suggestions or maybe have had and overcome this problem.Soundcard: Hercules Fortissimo IIThanks for any input,Pat