I realize this is a very old question by now but I'd like to share my experience.
I have a small bluetooth speaker that is about the size of a burrito. (About 8 inches by 2 inches | 20 cm x 6 cm).
I used to commute using a backpack and I managed to place the speaker inside the top of it, so the speaker was very close to my head. I used it at a volume such that if I needed to talk to/hear someone, the music was of little disturbance. In traffic, I could hear even the rolling noise of approaching cars, and in quiet streets it was actually quite enjoyable.
The downside, particularly for hearing speech is that any loud enough street sound would block the audio, making you miss some words. (Car horn, big engines, etc.) In my case, I used it for music and almost always I was hearing music I was already familiar with, so, missing a few beats was no issue at all. I was able to alleviate this somewhat by purposefully choosing more quiet streets, with the advantage of less car and pedestrian traffic, making the whole commute quite enjoyable.
This same speaker I also have mounted in my handlebar, which in this case sits almost right below my head, obviously at an arm's reach. This setup proved quite useful for a road ride in which I would spend quite some time far from other riders (due to performance differences). The same as before, the volume level was chosen such that I could hear even the less noisy cars approaching. In my case, this volume was not loud enough to be heard while away from the bike, but, for courtesy, If riding side by side or in a "peloton", I would turn the speaker off. You can test this by stepping a couple feet away, but consider that on the open street sound behaves very different from on your garage or underground parking.