There is a way to alter the behaviour of internal sounds via the config files of each aircraft. Well, I haven’t actually tried it but it should work.
Inside each aircraft package is a sound folder and inside that is config for how the game should interpret / utilise them. At the top of this config there is a section:
<AcousticParameters>
<Parameter Name="ReverberationPreset" ShareSetInside="rev_inside_default"/>
<Parameter Name="Insulation_ExitClosed" Value="-22"/>
<Parameter Name="Insulation_ExitOpen" Value="-12"/>
</AcousticParameters>
Increasing these numbers should increase the level of the engine sounds. I imagine they are decibel values and 0 (zero) would result in external audio being played at its maximum.
You can either edit these values directly which will require constant editing if updates replace the file or you can create a simple mod package that would replace this file in the games virtual file system. Making a mod package is not without issues either, you’d still want to check them after updates but it’s easier to manage.