Here, "들다" means "to go into a place",and the past form of it is "들었다". 들다 + 었(the past suffix) + ㄹ 적에 = 들었을 적에. "었" is added because the last consonant of the verb stem "들" is "ㅡ"(들 = ㄷ + ㅡ + ㄹ). \^o^/

듣다(v): hear, listen 듣다 > 들어 > 들었 (past)