I’m having exactly the same problem described here:
when reviewing, I have audio on the back of the card, and that audio plays with an initial fade-in effect. If I skip to the next card quickly enough, this effect won’t reappear, but if I wait a few seconds, then the next card will also definitely have this fade-in effect. This makes the beginning of the sound inaudible.
I tried a combination of audio settings in AnkiMobile (the “duck” setting) but nothing seems to help. I don’t believe AnkiMobile is doing something wrong, this is something happening between iOS and my headphones (Grado GW100). I guess they designed it that way because stop-and-go audio is a rare use case, and most people will be streaming audio or movies.
My workaround for now will be using wired headphones.