I had this issue come up in a recent RL game of mine and 2 of the players with Protection from Evil believed they were immune to the song during the duration of the spell. I let it slide because the song is described as a charm effect but I just wanted to know what the community thought.
Here are the relevant descriptions:
Captivating Song (Su)
A harpy's song has the power to infect the minds of those that hear it, calling them to the harpy's side. When a harpy sings, all creatures aside from other harpies within a 300-foot spread must succeed on a DC 16 Will saving throw or become captivated. A creature that successfully saves is not subject to the same harpy's song for 24 hours. A victim under the effects of the captivating song moves toward the harpy using the most direct means available. If the path leads them into a dangerous area such as through fire or off a cliff, that creature receives a second saving throw to end the effect before moving into peril. Captivated creatures can take no actions other than to defend themselves. A victim within 5 feet of the harpy simply stands and offers no resistance to the harpy's attacks. This effect continues for as long as the harpy sings and for 1 round thereafter. This is a sonic mind-affecting charm effect. The save DC is Charisma-based.
Protection from Evil:
This spell wards a creature from attacks by evil creatures, from mental control, and from summoned creatures.
Second, the subject immediately receives another saving throw (if one was allowed to begin with) against any spells or effects that possess or exercise mental control over the creature (including enchantment [charm] effects and enchantment [compulsion] effects). This saving throw is made with a +2 morale bonus, using the same DC as the original effect. If successful, such effects are suppressed for the duration of this spell. The effects resume when the duration of this spell expires. While under the effects of this spell, the target is immune to any new attempts to possess or exercise mental control over the target. This spell does not expel a controlling life force (such as a ghost or spellcaster using magic jar), but it does prevent them from controlling the target. This second effect only functions against spells and effects created by evil creatures or objects, subject to GM discretion.
I am honestly not trying to be rude but I am not even understanding where the confusion came from? The ability makes it nice and clear this is a mind-affecting !charm! effect and prot. from evil also makes it nice and clear this is exactly what it was designed to protect against.
I'm guessing you really wanted them to roll those saves and fail am I right? ;-) We've all been there. Hitting PCs with weird monster abilities is half the fun of DMing but when the PCs prepare properly or just get lucky you gotta let it go.
But remember Protection from Evil doesn't last for ever and as soon as it fails (assuming the PCs aren't constantly renewing it before it fail) the PCs will have to make the save if the song is still going on. Having been immune to it at the beginning does not mean that they made their save and are immune for 24 hours. If a harpy sees the song is obviously not working it might simply leave and wait for a better time. You'll get those pesky PCs eventually.
protection from evil only works on charm and compulsion effects where the caster is able to exercise control over the target, such as command, charm person, and dominate person; it doesn't work on sleep or confusion. (Sleep is a border case for this issue, but the designers feel that "this spell overrides your brain's sleep centers" is different enough than "this spell overrides your resistance to commands from others.")
I have seen some PFS DMs say they do not exercise direct enough control to count for this spell. Personally I strongly disagree that statement I have seen it. Especially since sleep is what they consider borderline on control, and harpy song is much more than sleep.
