Add multiplayer sound, so others can hear the engine

[hbeni]

Would be cool (and helpful) if the engine was also hearable from remote multiplayer planes.

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Ref:

• https://wiki.flightgear.org/Howto:Add_sound_effects_to_aircraft#Multiplayer_sound
• (unanswered topic) https://forum.flightgear.org/viewtopic.php?t=17546

[hbeni]

:information_source: Note, this does not work currently with FGFS 2020.4 because of a bug. See SF Ticket: https://sourceforge.net/p/flightgear/codetickets/2883/