It happens sometimes, when a real sound from the reality filters into the dream, and it is received unconsciously (a dream distracted by noise), in other circumstances – a warning. Such dream may also express the loneliness of the dreamer, and the need for social life.
hear: the announcement of visits
hear your own: a letter from a loved one
hear repeatedly: a pleasant trip
in the forest: solitude
in the mountains: a visit of someone nice