Headphones partially not working - not a hardware issue

Hey fellas,

so I recently got a used S6 Edge and there was a problem whenever I used headphones. The left earbud works fine but there is hardly any sound coming out of the right one(checked with several headphones, its a phone issue), really just minimal, very very quiet. I naturally figured this was a hardware error so I went to my local phone repair guy and had that part replaced.
Now, unfortunately the problem still isn't solved. With the new replacement it has gotten a little bit better, but the right earbud is still a lot quieter than the left one. So the repair guy said its a software issue.
Do you guys know anything about that?
I'm not rooted and on the official 5.1.1. Just tried factory resetting but that didn't really help. Any suggestions or anything I can look into that might solve this issue, be it rooted or not?