A SERIES of melodious Chinese and Western music pieces will echo in the City Hall on March 20 to usher in the freshness of spring. There will be two concerts in the evening, both at 8 pm. The Yao Yueh Chinese Music Association's ''A Concert of Works by Gu Guanren'' will be held at the Concert Hall while the violin recital by Leung Kin-fung will be in the theatre. Under the baton of Gu Guanren, who is also concertmaster of the Shanghai Ethnic Orchestra, the Yao Yueh Music Association will perform with zhongruan soloist Liu Bo and dizi soloist Chan Hung-yin. Tickets for both concerts from $35 to $60 are available at URBTIX outlets and reservations can be made at 734-9009.