That depends a great deal on what the navigation wearable is suppose to do. If it is suppose to give someone turn by turn directions of a long, complex routes with street crossings and complicated turns, voice would be necessary to give those kinds of directions.

Sound is quite common, but you don't want to drown out things you need to near (such as traffic). Even with binaural headphones, it may also not be precise enough for what angle to walk (although often good enough). Also, depending on the age range (older people are more likely to have hearing loss), they may not hear well enough for that.

Vibrations can be trickier, because it can be harder for gradiations of the same degree as sound. Most devices I know that use vibrations only have two vibrations setting usually, one weak and one strong.