Question

i must have been sitting in dark corner.my voice startled her(join into a complex sentance)

Answer 1

The answer is: As my voice startled her, I must have been sitting in dark corner.

Here the principal clause is I must have been sitting in dark corner, and As my voice startled her clause is added to this sentence as an auxiliary clause for the expansion of the sentence.

Here meaning of the sentence has been more clarified.

Answer 2

Answer:

I must have been sitting in a dark corner when my voice startled her.