Is the programmer sinful if he facilitates the user's browsing, listening to, and downloading of audio and video files from the server to the mobile phone, knowing that these files may be songs and that the users are from foreign countries?

It is permissible to work in providing people with a service that can be used for either permissible or forbidden matters, except for one who is known or it is highly probable that they will use it for forbidden purposes. In such a case, it is forbidden to provide them with it. If the majority of the program's users are disbelievers and it is highly probable that they will use it for forbidden purposes, then it is not permissible to work for them unless it is stipulated to them that they avoid listening to what is impermissible and they agree to this. This is because anything intended for forbidden purposes, or where it is highly probable that it will be used for such, is forbidden to manufacture and sell. Consequently, the ruling on developing programs depends on the users' adherence to the condition of not using them in what violates Islamic law. If they adhere, it is permissible; if they do not, it is not permissible. If there is no high probability of either, the default ruling is permissibility, and their affirmation of agreement to the condition suffices.