I am just trying to retrace your steps - you thought you might check if the phone had a hardware issue by seeing if sound was fine on Stock ROM.
I think that to complete that bit of research through “forced” Samsung channel, you would be forced to boot into latest Android Stock ROM.
If you thought “can I just see what the sound would be like on verified_by_Samsung, Oreo?”, the answer is
No - we can only serve you latest due to Rollback protection
Here you mention that you can get heimdall to cooperate in producing a solution where you avoid verified boot.
That is great.
I think to meet the requirement to install a “Stock ROM”, the ROM (probably) must be complete and full, in order to meet the “Verified boot”?