Q. Can a woman sing in her home if they are Jewish workers present engaged in their work and are not listening to her?
Similarly, can a group of female students sing while they are traveling on a school bus when the driver is Jewish?
What if the workers are Jewish but not frum at all.

A. Halichos Shlomo (1: 20: n. 20) rules that a woman should not sing in the presence of Jewish workers even if they are engaged in their work. The heter permitting sexually arousing activities for people occupied in their occupation or business, such as doctors or women-clothing dealers, namely “beavidetei torid” (Baba Metzia 91a, Avoda Zara 20b), does not apply, since from time to time, they will halt their work and likely listen. The same would refer to a bus driver.
Kinyan Torah (1: 20) similarly prohibits women singing in a theater or hall, even if it is only in the presence of other women, if the sound technicians and other workers are Jewish.
Ohm Ani Chomma (1: p.71) maintains that the above does not apply if the workers are Gentiles, since they are not included in these type of prohibitions.
If the workers are Jewish but not religious, Horav Shlomo Miller’s Shlit’a opinion is that in need it may be permitted. This being similar as students singing in the bus or in school, when walkers by are occasionally listening to them. The Rov also pointed out to the admissibility of women washing clothing by the river. (ibid.).

Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit’a