I personally don't see this addition being added specifically in TruckersMP. Especially because this sound effect would only play for yourself. Other players wouldn't be affected by this when you complete a job.
This suggestion would probably be better for SCS themselves, the creators of this game. TruckersMP is only an unofficial multiplayer mod and whilst we do use some custom mods, these are limited in what they can do.
If you'd like to create a suggestion for ETS2, you could do so at https://forum.scssoft.com/viewforum.php?f=5
Or for ATS at https://forum.scssoft.com/viewforum.php?f=185
For their forums, do keep in mind their guidelines when creating a suggestion: https://forum.scssoft.com/viewtopic.php?t=76526