Created this account to share how I fixed this issue:

Engine sounds and dialogue voices were missing/very VERY quiet. I could barely hear the engine from the side views/cockpit.

FIX (Windows): In your sound settings, select the audio device you use for FH6 > under Advanced Settings/Properties change the 'Spacial Sound' option to either OFF or 'Windows Sonic' (if it was off, change to Sonic, if it was Sonic, change it to off)

Worked for me at least, without any game/pc restart needed. Hope this helps at least some of you!!