the way my brain works, it helps to see a particular bird actually making a particular call. so i just try to get out with the birds when they’re calling and singing and try to see them actually make the sounds they make. i’ve also found that making videos of the calls is sort of a force multiplier because i can go back home and watch a particular call or song as much as i want whenever i want. i find that my own videos are often better for learning than some of the generic ones i find online – even if the ones online are better quality technically – because my videos are guaranteed to have songs from birds in my particular area (ex. the Carolina Wrens in my area have abbreviated songs that none of the popular online guides note), and they also tie back to a previous actual experience i had.
going out birding with a group (such as a local bird survey) led by a birder who is really good at birding by ear can help, as can using a sound recognition app like Merlin, but i think of these methods as most helpful when you’re just starting to learn the birds in a particular area and need help figuring out what’s interesting and what’s worth prioritizing effort to look for. but, for me, even once i’ve been told what made a particular sound, i still want to make an extra effort to see the actual bird calling or singing to really cement a bird sound in my brain. once i’ve learned a lot of bird sounds in the area, then i’m more able to prioritize myself just based on “what is that sound that i don’t recognize?”.