I would agree if you have already tried both of those steps it would definitely be a hardware issue at that point. To resolve your issue you will need to have your phone looked by a technician. You can use this link here to find one near you. http://bit.ly/2HP0oRi
Let's try placing the phone in safe mode? It would be a third party app causing the issue. This is how you place the phone in safe mode link.
Is this some type of joke? User asks for help... given multiple links that are all broken and then the marked Solved answer is his own reply saying that it isn't a Media volume issue, it's an in call volume issue. SMH
This is an issue most users are having since Pie update. As usual... Pie is to blame. My entire phone has suffered low volume across the board. In call volume is low and have to jack the volume to the max which I didn't have to do with Oreo and same with media volume which also has to be jacked to max, where as in Oreo I could have it at around 3/4 volume... which was still sad, but nowhere near as sad as it is now with Pie.