The volume audible outside is comparable to that of in-ear headphones with foam rubber pads. In the fitness centre, with the background noise, you can't hear anything at a distance of 1m; but in the quiet compartment or library, you can. When worn normally, the treble is stronger than the bass and the sound is surprisingly clear. Ideal if you want to listen to a podcast while jogging or cycling (or gardening etc.). You can hear the traffic around you perfectly well (but hardly hear the podcast in subways or overtaking lorries). If you use earplugs (included), the bass is stronger than the treble, but you can't hear your surroundings as well. What's very good is that it doesn't fall off even when you're shaking or sweating.
The sound is also very good when lying sideways on a pillow; when lying on your back, you may have to fold the headband up a little so that the headphones are not pushed forward.
The headset function is a nice-to-have when a call comes in while jogging, but since the microphone is at the back of the ear cup, it's no substitute for a proper headset. In wind from 20 km/h (e.g. cycling), the other person only hears noise.
As a gimmick, the headset can be clamped onto a guitar (sound body) as a loudspeaker.
I myself am satisfied with the device. (Of course, it would be perfect if a DAB+ radio could be integrated in the future, as long as the headband is sufficient as an antenna - maybe I will be listened to...).

I think every piece of fitness equipment, such as an ergometer or cross trainer, is louder than what comes out. You can hear it when it's very quiet. Of course, your ears are free and you can hear the ambient noise. They are not quite as loud as conventional earplugs.