Fish Tank

Mia Williams is a volatile and socially isolated 15-year-old. She lives on an East London council estate with her single mother, Joanne, and younger sister, Tyler, and is highly antagonistic toward both of them. Mia is a loner, appearing to have had a falling out with her best friend Keely. She provokes Keely's other friends and head butts another girl when Mia criticises their dance routine. Mia regularly practises hip-hop dance alone in a deserted flat.
Near the estate, Mia comes across a skinny, tethered horse in a Traveller encampment. She tries to free it, only to be caught, taunted, and assaulted by two young men, the horse's owners. A third young man, Billy, the brother of the other two, is more sympathetic. He shows kindness towards Mia and explains that the horse is old and ill.
Joanne's new boyfriend, Conor O'Reily, is a charming and handsome Irishman. He drives them on a day-trip into the countryside, intent on making a good impression with the daughters. He introduces them to his favourite song, Bobby Womack's version of "California Dreamin'", shows Mia how to catch a fish using noodling, and asks her to show off her dance moves. Although Mia is abrupt with Conor, she appears to be intrigued by him.
Mia goes to a cybercafe and watches amateur breakdancing videos on YouTube. Just as Mia is about to leave, she finds a flyer for a club seeking dancers and at the door, Keely's friends enter and they exchange taunts. Mia visits Conor at work, as a security guard at a hardware store along with Billy, and they sneak into a junkyard for auto engine parts. Conor encourages Mia to apply for the dancing audition. He lends her a video camera to record an audition tape. Their interactions continue to be flirtatious. After giving Mia the camera, Conor puts on cologne and leans into Mia while asking what she thinks of it before suddenly administering a spanking for running off when a social worker visited. One night Mia secretly witnesses Conor and Joanne having sex. Mia then goes back to her room and slams the door several times. Mia sends in her tape and is invited by the club to perform in person. Late one night, with Joanne passed out drunk upstairs, and after Mia and Conor have both been drinking, he asks to see her dance routine in the living room. When she dances to "California Dreamin'", Conor then invites her to sit next to him which leads to them having sex. Before returning to Joanne's bedroom, Conor tells Mia to keep their liaison a secret.
The following morning, Mia hears her mother crying, Tyler tells her that Conor has left. Mia tracks him down to his home in Chadwell St Mary and confronts him. He explains that he cannot see Mia any more because of her age. He drives her to Tilbury Town railway station and provides her fare. Instead, Mia comes back and sneaks in through the house's back window. Finding a video camera and watching it, she then discovers Conor's wife and young daughter, Keira. Mia then urinates on the living room floor and sneaks out of the back door upon the return of Conor's family.
Watching Keira riding her scooter on the house's road, Mia impulsively pressures Keira to go with her under the claim that Keira's mother told Mia to take her for ice cream. Going out into the fields and reaching the River Thames, Keira tries to escape. Mia catches up with her, but in the struggle, Mia throws Keira into the turbulent river water, disappearing and resurfacing after a moment then Mia pulls her out, and takes her home as night falls. When walking along a street, Conor's car screeches to a halt beside her. Mia leads Conor on a chase, when he catches up with her, he slaps her, knocking her to the ground, and then walks away without saying a word.
The next day, Mia goes to her audition. It is immediately obvious that it is for erotic dancers. The other participants are all grown women wearing heavy makeup and provocative clothing. Mia takes the stage wearing a hoodie, but as the music starts, she walks off before performing her routine.
Mia goes in search of Billy. When she arrives at his place, Billy tells her that the horse had to be put down. Mia sinks to the ground in tears. Billy says he is moving to Cardiff, Wales, and invites her to join him.
Mia returns home to pack, dances with Joanne and Tyler, and goes to Billy's car after hugging Tyler goodbye. The two set off for Wales.

Question: Who does Mia hear crying?
Answer: her mother