You can set music files as ringtones using either Settings->Sounds and Feedback, or Settings->Ambiences->(select an ambience)->Add Actions->Ringtone.
The latter method will set that tone only while that Ambience is in use, or until something else changes it again (you, or another Ambience).
If using a dual-SIM phone, there should be an option to set a different tone for each SIM, but I’m not sure where that setting is.