The downside of the game lasting so long and supporting older content, is that major changes like the new sound engine won't be fully supported by all content. The new sound engine is a huge step forward, and IMO is worth the issues with some older content. That said, how much of the troublesome content will never be updated? i.e. the old ISI/S397 out of license cars that probably won't ever be refreshed; everything else has the potential to be fixed.
I think you are right, despite I noticed a "yerrible" (at least for me) rumble sound always present in background, a sort of unfeleting road noise for my old favourite cars: Renault Megane 2013, Dallara dw12 2014, Renault F3,5, Tatuus F3-020 In particular I' m a fan of downshifting sounds, and with the old engine sound, the megane is a joy to listen at every downshift (bam, bam,bam...Great !) With the new engine this disappears almost in toto. Speaking in general for all others cars the inside sounds is always affected by this rumble (sorry but english is not my native language and I am not sure the word rumble is just what I want to describe). Anyway thank for the replay and I will continue to use the old engine sound (sbam, sbam, sbam...