Ideally, there would be no need for feedback when we meditate. We should be able to notice when our mind is focused and when it is distracted. But this requires practice, and, even with practice, it is sometimes difficult to maintain the appropriate focus and awareness. So this is where devices such as the Muse might prove helpful.
However, the biggest issue with using feedback technologies with meditation is that certain types of feedback can actually interfere with the activity they are designed to assist.
Let's assume we are doing focused attention meditation on the breath.
If I am remaining focused and not wandering, having any kind of feedback is likely to interfere with my focus. If I am truly focused on the breath, I should be aware that I am focused on the breath. I really don't need positive reinforcement. On the other hand, if my mind is wandering and not focused, I may need to be alerted to this fact, reminding me that I should resume attention to the breath. Over time, I may become more sensitive to the wandering and more able to readily return to the breath without the warning.
Auditory feedback is a readily available form of feedback, especially when we are already listening to instructions from the app via headphones or speakers. However, it is common sense that attention to external stimuli such as sound will conflict with attention to the breath. Whereas attention to the breath activates the interoceptive cortex (Farb et al, 2013), sound activates the auditory cortex. If my mind is wandering anyway, this auditory "distraction" should not pose a great difficulty. However, it should be momentary and only repeated long enough for me to recognize that I must redirect my attention. If the auditory feedback is anything more than a quick reminder, it is likely to become a problem.
The Calm app provides different forms of auditory feedback. Both the wind and the waves are giving feedback about the degree to which the mind is active. The wind supposedly represents current activity, whereas the waves represent activity over a more extended time-frame. Both the wind and waves vary in intensity and are continuous rather than discrete sounds. The birds are supposed to represent a calm mind, and they reward you for maintaining calm over a period of time. If I hear any of these sounds, they are very likely to become items of interest in themselves, thereby distracting me from focusing on the breath. Also this kind of feedback is likely to stimulate evaluation and judgement--"I am doing well," "I am not doing well," and so on.
Whether different forms of feedback would make a difference or not remains to be seen. Haptic feedback (see http://www.arcwearables.com) might cause less conflict than auditory feedback, for instance. On the other hand, with auditory feedback it might be possible to provide more differentiated feedback that sensitizes the user to the type of activity that is going on. In which case the feedback would not so much function as a warning (wind and waves) or as positive reinforcement (birds), but as information about what is going on.
These issues challenge the concept of using a device such as this as a meditation aide. It is to be hoped that appropriate research will be done to determine whether the Muse (and other devices like it) can be useful for the purpose of assisting meditation.
Farb, N. A., Segal, Z. V., & Anderson, A. K. (2013). Attentional modulation of primary interoceptive and exteroceptive cortices. Cerebral cortex, 23(1), 114-126.