I know this is a touchy subject, but I feel like ranting, so here are my thoughts on the subject:

I don't want to hear what your taste in music is while we're in the boulders. Especially through small, tinny speakers, and definitely not without you asking me (who was there already) first.

I've gotten a bit riled up this evening.
I went out to the boulders after a day of work to do a quick easy circuit and a small group of boulderers that showed up after me decided to play music loudly through a small speaker attached to their phone.

I didn't feel like being confrontational, so I just left, but it left a sour taste in my mouth.

If you're the type of person that sometimes does this... please consider that maybe the people around you don't actually want to listen to your music, but they just don't feel like telling you to cut it out.

At least ask if it's ok.