What is the opinion of Reddit about the
UE ROLL Wireless Mobile Bluetooth Speaker (Waterproof and Shockproof) – Volcano?

My Dad gets great use from his UE Roll, for somewhat less cost than the Boom 2. Stays in his saw dust-filled workshop day-in-day-out, and still works great 9 months later.

There are some bluetooth speakers that can attach to different things, I have these, and they would probably work. I would only use them on very tame trails.
http://www.amazon.com/UE-Wireless-Bluetooth-Waterproof-Shockproof/dp/B00YARCGHO