MB 17: A woman singing - Even if she is unmarried, but if he is not reading the Shema [or any other prayers] it is permitted provided he does not intend to get enjoyment from [her singing], in order that he should not be led to have lewd thoughts. But the singing of a married woman (and also that of any other woman who is forbidden to him [eg. his sister]) is always forbidden for him to listen to, and an unmarried woman who is in a state of Niddah [ie. has menstruated and has not been to the Mikveh (ritual bath) to cleanse herself] is considered as one who is forbidden to him for these purposes; and nowadays [because women do not go to Mikveh until they are married], all young girls are assumed to be in a state of Niddah once they each the age of maturity.
The singing of an unmarried non-Jewish woman is also considered as "nakedness" and it is forbidden to listen to it whether he is a Kohen or a Yisroel. Nevertheless, if he is traveling among non-Jews or he is in a city and he is in an unavoidable situation where it it is impossible for him to stop them, since we do not find that [a non-Jewish woman's singing] is called "nakedness" from the Torah point of view it is permitted for him to read the Shema and make Blessings, because were this not the case [that we permit him to read the Shema etc.] since we live among non-Jews the Torah and Davening would be abrogated, and on this it is said [in Psalms] "Now is the time to act in the L-rd's cause; they have destroyed your Torah", but he should force his heart to concentrate on the holiness that he is involved in and not pay any attention to the singing.
MB 18: Speaking voice - Meaning that because one is used to hearing women speak, he will not be led to have lewd thoughts, and even from a married woman. Even so, it is forbidden to listen to her speaking voice for enjoyment, because it is even forbidden to derive enjoyment from looking at her clothes.