Listen to a short clip of the sounds of a pedestrian underpass in Beijing.
Beijing has many road underpasses for pedestrians to use as they negotiate the busy streets.
One can usually expect to find various entrepreneurial people selling their wares. Occasionally there may even be buskers playing some sort of music but selling various goods is a far more popular past time in such places. Goods for sale include things like children's clothing, cushions, flowers, fruit, toys, hair clips and other cheap adornments to wear or decorate the car or phone.
If you decide to buy from these people be sure to bargain for the goods, don't pay the first price they quote.