Is it permissible to enter weightlifting gyms that play music containing obscene language, in order to strengthen the body for self-defense in a country where theft and harassment are widespread, especially knowing that there is a sister who needs to be defended?

It is not permissible to go to a gym that contains music or other forbidden things. The argument of strengthening muscles for self-defense and defending one's family is not religiously justified, as this can be achieved by exercising at home or in other places free from forbidden things. Indeed, Allah's protection is attained by observing His commands and prohibitions, obeying Him, and staying away from disobeying Him, as the Prophet, peace and blessings be upon him, said to Ibn Abbas: "Be mindful of Allah, and Allah will protect you. Be mindful of Allah, and you will find Him in front of you."