All good points. Let's plug the easy stuff into homebrews and then work on the song.
Off to a good start!
I believe some of these abilities have been covered by some of the critters in Book of Exalted Deeds. I'll have to look at 'em when I get a chance.

Sorry it took so long. It didn't pan out quite as well as expected.
The song is somewhat similar to bardic music (fascinate, for example) so we could build from this:
Song (Su): A firre has a captivating voice and can use bardic music just as a bard can, inspiring courage, fascinating, inspiring competence, or giving suggestions to those who hear it (see Bardic Music in Chapter 3 of the Player's Handbook). Unlike a bard, however, a firre can sing as often as it likes.
The plane shift ability sounds like the limited plane shift ability of genies, but rather than the Elemental Planes, it can go to and from any plane that touches the River Oceanus. It also sounds a bit like ethereal jaunt of the stalker/ficher/marauder, once again with different planes involved.
That doesn't resemble the Delphon's song, I think we need something similar to the confusion spell.
The song of a dolphin is so alien and mysterious to other creatures that it has profound effects on them. If an intelligent creature hears the keening voice of a delphon, a saving throw versus paralyzation is required to avoid the effects. If the saving throw is failed, roll 1d12 to determine what happens to the listener.
D12 Roll Effect
1-4 Listener is charmed for 1d10 hours, enamored by the sound of the sweet melodies.
5-7 Listener is affected by confusion for 3d6 rounds, unable to comprehend the alieness of the bizarre sounds
8-9 Listener is spellbound (held) by the sound for 1d10 rounds
10-11 Listener is rendered unconscious for 3d10 rounds, unable to handle the mind-shattering notes and their profound meanings
12 Listener is struck by the sound so completely that he begins to learn the the meaning of the song, and suddenly understands something new about the multiverse
Those that make a successful saving throw hear nothing more than odd, discordant music. Creatures that avoid the negative effects of the song gain a bit of planar knowledge about travel on Oceanus, the relationships between the planes (especially those that Oceanus touches), or places on and around Oceanus. In game terms, characters that fail the saving throw but roll a 12 on the die may ask the DM one question pertaining to Oceanus and its environs. Some such questions include: the location of the nearest gate to a neighboring plane; how to reach the Isles of the Blessed in Thalasia; whether or not the River Oceanus is truly beginning to “leak” over into Ysgard through the Gates of the Moon; or how to avoid the natural hazards of travel on Oceanus.
Nonintelligent listeners are always charmed by the delphon’s song. Eventually, intelligent beings that spend time around the dolphins grow immune to the strange consequences of their songs, learning to more fully understand their deep and complicated meanings.
I was thinking of an Ethereal Filcher cum Blink Dog approach, since it can avoid attacks and get behind enemies with it.
Hmm...I think that is part of it, but it almost more like a prismatic spray, or maybe even like a holy word with greater effects on weaker creatures.
To summarize:
Creatures of less than Int 3 are simply charmed.
The random effects:
- Charmed for 1d10 hours
- Confused for 3d6 rounds
- Held or maybe fascinated for 1d10 rounds (I prefer the latter)
- Rendered unconscious for 3d10 rounds
- Gain a free Knowledge (the planes) check with a bonus (or equal to the delphon's modifier?)
That looks about right. I suggest adding a time limit to the Knowledge check bonus.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.