It's not hard to understand my friend's frustration. Having your earbuds in is the universal sign for "leave me alone." Regardless of your gender, if you spend much time out in public by yourself, you've probably done it. Maybe you were taking the train and wanted to avoid a chatty seatmate, or, like my friend, you were hoping to sip your joe in peace. If you're a woman, you may have an additional reason for putting your earbuds in...attempting to proactively fend off unwanted advances. However, as my friend learned, it's a tactic that has a fairly high probability of backfiring: Listening to music, an audiobook or a podcast takes one of your most important senses—your hearing—out of commission.
When you're learning about self-defense, you'll no doubt come across mention of Col. Jeff Cooper's "Color Code." In short, it's a way of thinking about levels of awareness of our surroundings. Ideally, we should be spending the majority of our time in "Condition Yellow," which is essentially a state of relaxed alertness, and only going "Condition White" when we are asleep. When you have earbuds in your ears, unfortunately, although the rest of you may be "Condition Yellow," your ears are "Condition White." To return to my friend's story, chances are quite good that if she had not been listening to her podcast at all (or if it were coming from a source that didn't block other noises, such as the coffee shop's stereo system), she would have heard the interloper's approach before he got to earbud-yanking range. He may well still have tried to chat her up—this person was clearly bent on ignoring social norms anyway—but it wouldn't have caught her unaware.
There's another reason to reconsider using your earphones in public. Have you ever, while driving, turned off your radio while you were looking for an address? It seems like a setup to a joke, but many people find that it's easier to concentrate and process visual information when auditory distractions are reduced. Of course, the extent to which this applies varies from person to person, as some of us are better multitaskers than others. The nature of what you're listening to and the volume at which it's being played also factors in to how distracting you'll find it. But even if you're a terrific multitasker, you only have about a 180-degree field of vision, so why block the sense that alerts you to things happening behind you?
Finally, as we've mentioned before in this space, one of your goals should be to fail a predator's "interview process," and a key part of that is to simply not look like prey. An ideal victim is unaware of her surroundings and distracted, so don't allow yourself to look distracted and unaware. Unfortunately, there are people out there who want to do far worse than yank your earbuds out and ask for your phone number.