According to the Shulhan 'Aruch (Even Ha'Ezer 21:1), it is:
ואסור לשמוע קול ערוה או לראות שערה. והמתכוין לאחד מאלו הדברים, מכין אותו מכת מרדות. ואלו הדברים אסורים גם בחייבי לאוין.
Forbidden to listen to the voice of an 'Ervah or to see her hair, and we strike someone who does one of these things Makkath Marduth, and these things are also forbidden with (regard to women in the category of) general [i.e. not Kareith] prohibitions.
As explained by R' Eliyahu Reingold (from just before the 28 minute mark in this Shi'ur), the point of this language of "general prohibitions" is to include even a lesser classification of the prohibition of Niddah (possibly non-'Ervah) in this prohibition.
But what was left out intrigues me.
Makkath Marduth is a punishment that is usually given to someone who deliberately violates a rabbinic prohibition. Does this mean that the Mehaber holds that listening to Kol Ishah and looking at a woman's uncovered hair are rabbinic prohibitions?