The subject of your sentence can be something other than the MC when you're writing in first person. For example:
My phone rang
Footsteps were coming down the alley behind me
You don't need to always narrate in the active: "I saw", "I heard", "I conquered". You don't even need to specify your character's presence - it's assumed. My first example could just as easily have been "the phone rang".
There is, however, a crucial difference between my second example, and your example. When you say "were heard", there's the immediate question "by whom". Since the story is told in first person, this question should never arise. The answer is always "I - the narrator - heard". On this account, your instinct was right. But if the footsteps "were coming", then it's implied that the narrator heard them, there's no need to specify "I heard footsteps coming."