Hi,
Personally, I downloaded a 'FIX' for that but I can't say where it comes from, I'll share it with you, it's hosted on my server, it activates like a 'MOD' WARNING! It should not be left active !!!
https://www.fsx3d.com/FIX_SOUNDS_AC_HOLD_MODS.zip
With this FIX I have the exterior sounds on the 787 and other 'HOLD AC' cars
Have fun ;)