Ghost Sound saving throw includes "(if interacted with)". I had assumed this meant "if you hear it" in which case all folks would get a save, however, it was pointed out to me then why have this in there? Is there something I'm missing which you would not get a saving throw to Ghost Sound and still hear it?
There are 5 sub-schools of the Illusion school:
- Figment: A figment spell creates a false sensation. Those who perceive the figment perceive the same thing, not their own slightly different versions of the figment. [...]
- Glamer: A glamer spell changes a subject’s sensory qualities, making it look, feel, taste, smell, or sound like something else, or even seem to disappear.
- Pattern: Like a figment, a pattern spell creates an image that others can see, but a pattern also affects the minds of those who see it or are caught in it. All patterns are mind-affecting spells.
- Phantasm: A phantasm spell creates a mental image that usually only the caster and the subject (or subjects) of the spell can perceive. [...]
- Shadow: A shadow spell creates something that is partially real from extradimensional energy. [...]
If you look at the entries of other Figment spells, such as 1st-level Silent Image or 6th-level Mislead you will note that their Save field share the exact same text: Will disbelief (if interacted with).
I expect that most, if not all, figments have the exact same text; copy/pasted.
With regard to this particular spell, I would note that a Deaf character, by virtue of not hearing the sound, cannot interact with it, and therefore cannot disbelieve it.
This may seem obvious and pointless, until you realize that said character was the most likely to succeed and now cannot then first succeed and then help others disbelieve:
A failed saving throw indicates that a character fails to notice something is amiss. A character faced with proof that an illusion isn’t real needs no saving throw. If any viewer successfully disbelieves an illusion and communicates this fact to others, each such viewer gains a saving throw with a +4 bonus.
Note-to-self: render the Cleric and Druid Deaf before using Ghost Sound on the party.
Hearing a sound is interacting with it. There are other ways you could interact with a sound that don't involve hearing. A sound will create ripples in a still pond. A very loud sound is accompanied by a noticeable sound pressure wave. Noticing the lack of these impressions would also be interacting with the figment.
The text from Ventriloquism, also a figment, implies anyone who hears rolls a save.
With respect to such voices and sounds, anyone who hears the sound and rolls a successful save recognizes it as illusory (but still hears it).
It means that you have to perform an action...
The answer to this is actually in the Player's Handbook, p173, under the Magic Overview section: Saving Throws and Illusions (Disbelief):
Creatures encountering an illusion usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion.
In the case of figments, the 'if interacted with' clause is definitely sticky. What it means is that a character has to perform some action, even just looking or listening harder, in order to get a saving throw. You either take an action to look or listen harder at the figment of the Illusionists imagination, or you otherwise interact with it. In the case of visual illusions, taking an action that might prompt a response from a represented creature (this could even be a free action like talking to it), or examining an illusory door for traps count as interaction. In the case of auditory only illusions, listening harder is nearly the only action that would provide a save, though there can easily be exceptions.
Merely seeing or hearing a figment, without performing some other action, generally will not provide a saving throw, unless the spell says so.
As DM, I usually give saving throws to anyone who threatens illusory creatures or would be threatened by them, and also to anyone who takes an action to listen or spot other valid illusions. Just talking to a visual figment or even saying 'Hello?' upon hearing a ghost sound could count as interacting with the illusion.
Unfortunately, just as with illusions themselves, there's a lot of room for adjudication, and it's really up to how the DM, and the group, sees things.