Ils sont and ils ont are almost indistinguishable on audio without context!
listen again, you should here
ils sont = IL-SS-ONT
ils ont = IL-Z-ONT
In a strange accent that difference is very hard to hear!
It is just something that comes with practice. Clever programmes like Duolingo may be invented to teach us language efficiently, but practice is still necessary to understand the many variations in pronunciation in the real world.
But you never know who you'll talk to, so it's good to get used to all sorts of accents!