Ave Sodales,
Does the Seirenes' supernatural Virtue (which grant them the power to control whoever hears their music performances) make their Music ability to be aligned with a supernatural Realm ?
Or does their ability to control humans stay pure mundane effect, making their music fully operative even in a Dominion aura ? As far as I can see, a Seirene can bewitch the Holy Pope standing in the Saint Sepulchre as easily as a faery queen in her floating castle, or a poor beggar in a wild forest.
It is a non mundane ability, like enchanting music it would have to penetrate ?
No, they use the "Music" general ability.
The Seirene's virtue is treated like any other in HoH:Societates. See p104 "It is assumed that for magi Ex Miscellanea, this ability is affiliated with the Magic realm, although it can be associated with any of the four supernatural realms, creating a character with multiple affiliations (since The Gift itself is always aligned to the Magic realm.)"
Playing a normal melody will be a mundane use of music. Playing a tune to use "Siren song" uses their Major Virtue, so is magical and is affected by realm effects normally.
Now it makes sense.