I think I listen something = Думаю что я услышал что-то
Услышал = heard (Past Indefinite)

Слушал (actively) = listened to something
Слышал (passively) = heard something

I think I listen to something = Я думаю, я что-то слушаю.
Sounds very awkward as if the person speaking isn't sure whether he listens to something or not; especially given the Simple Present refers to regular, repetitive, habitual actions ongoing through days and weeks. How can one be not sure whether he listens to the radio e.g. on weekends or not?