The song of the young girl reaping in the fields is incomprehensible to him, and what he appreciates is its tone, its expressive beauty, and the mood it creates within him, rather than its explicit content, at which he can only guess.

在这段话中，你的两个问题分别是：

1. within him：在他身上；在他内心中

2. at which 不可改为in which。因为guess at是固定搭配，意为：猜测，估计。