Well, then you don't *need* an interface. As everyone has pretty well said, you can go from the 3/8" (3.5mm) jack to a split-out that can send an unbalanced 1/4" cable to the JBL's input, ignoring the "balanced" label there. Heck, you can even run speaker wires, apparently!
You do give up the headphone option doing that however, so *IF* you think you'll be plugging/unplugging that adapter with any regularity to switch back and forth to headphones, something like the Behringer UCA-202 might save you needing to do the switching and keep the jack from becoming a failure point. (I used an earlier version of that adapter, UCA200, that's 12 years old and still works! Can't for the life of me explain why it's still around
)