If you want people to listen, don’t shout; whisper. That might be the thinking behind the music of Miroslav Srnka, a young Czech composer who appeared at the Phillips Collection with the Fama Quartet on Thursday night to open the gallery’s Leading European Composers series. Srnka presented three of his recent chamber works for strings, intriguing pieces that explored a sound world of spare, often rough textures and fragile sonorities — and rarely rose above a delicate pianissimo.