Well, I just bought my ZenFone 6 and need to say that I am also disappointed with its speakers. I loved its camera and how fast it is etc. However, when I watched the first video in YouTube I quickly realised that something was wrong. I even though my ZenFone 6 arrived with defective speakers, then I found this post. It is pretty easy to notice the bad stereo environment. Simply watch one of those videos for testing the left and the right speakers in YouTube.
I am disappointed because the sound quality is much lower than in my old cell phone, which is 6 years old (LG G3) and my girlfriend's phone, which is almost 1/3 of ZenFone 6 price (Motorola Moto G6 Plus). As a suggestion, I am sure that being able to equalize or/and adjust each speaker volume independently, or at least being able to panning stereo audio, would improve this. I hope my comment can somehow be useful to you. All the best!