The easiest way to start birding by ear isn't a recording. It's your own yard. Begin with birds you already know by sight. While you're watching one, listen at the same time. That connects the sound to the bird firsthand, instead of learning it from an app or a recording with no context attached to it. Add one new song at a time rather than trying to absorb several at once. And keep coming back to the same spot. Repetition is what makes a song stick. I once watched a Carolina Wren sing in my backyard. Watching the bird as it sings is a great way to have it stick in your memory. (since then, I've never forgot what a Carolina Wren sounds like!)