Answer: The sound of a woman's singing and playing
Step-by-step explanation:
In this excerpt, the speaker wishes to repeat the singing and playing of a woman, as is clearly stated in the following lines:
Could I revive within me
Her symphony and song,
To such a deep delight ‘twould win me…
The reason why the narrator wants to repeat her singing and playing is that, to that sound, he would "build" in air the dome he once saw in a dream. The poem is, by the way, based on that dream. In it, a man called Kubla Khan arrives at a dome where nature inspires fear and wonder simultaneously. Ice caves coexist with brightness and sunshine. Voices talk of prophetic war while gardens blossom full of incense trees.
The speaker's identity seems to melt into Khan's, and he desires to relive that dream, to build that dome in the air, to feel what he once felt.