"似笛声的" can be translated into English as "resembling the sound of a flute" or "like the sound of a flute".
For example:
The melody was resembling the sound of a flute.（那旋律似笛声。）
The wind whistled through the trees, creating a sound like the sound of a flute.（风穿过树林，发出似笛声的声响。）