> Not everyone owns headphones. Some people might have received the speaker as a gift or decided on the speaker instead of headphones. How people spend their time outdoors is not up to you or I to decide. If they want to listen to music from a bluetooth speaker, that's what they want to do. There's a lot more outdoors for you to use as well so rather that stewing, just find more outdoors. Especially on trails. Just keep going. Or wait until they have kept going. I've never seen a bluetooth speaker that's big enough for someone to be on a trail with that doesn't "go away" after a minute or so.
I am very open to the argument of "you do you", which is pretty much my philosophy also. But I do think there are /some/ limits to this, because some behaviors are inherently anti-social. My philosophy is more than "you do you" should apply to policy and regulation, meaning that we should not criminalize or directly punish anti-social behaviors that don't cause direct and immediate harm. But that definitely does not mean that we should not shame people for acting in completely inappropriate ways, or directly inform them that their behavior is unwelcome, or otherwise seek to ensure that we act to exist in spaces devoid of anti-social behavior.
I've had this same exact scenario happen, and I simply spoke to the person and told them to lower the volume, use headphones, or stop altogether because they were scaring away the wildlife that I was there to see and photograph. They apologized, lowered the volume, and we both went back to doing our own thing. Most people are reasonable, and act in anti-social ways due to lack of awareness not malice. We are both sharing the trail, and we are both there to experience nature, and that very well might include many different modalities (including accompanying music), but if someone is acting in a way that completely prevents me from enjoying nature I definitely have the right to say something, to complain about it, and to complain about it after the fact, and "you do you" is not a valid argument in response to that.