I am using Galaxy J8. If I play slow motion videos which are created on s7 or s8 they plays perfectly fine. Then why you are not giving Slow Motion Mode in galaxy j8. Please add this option in next firmware update.
And this happening when playing via Phone speaker or headphone/bluetooth earbuds ? If it is happening when playing via Headphone/bt earbuds. Then check Media Volume Limiter may be it's On (Turn it Off) if it is happening when playing via phone speake...
