Dolphins use such high-pitched and information dense communication that it's highly possible they are communicating raw data that is directly converted into a 3d map or image by the receiving dolphin's brain. Listen to a dolphin's speech, then think of what a 56k modem sounds like... not too far off huh? Now imagine that dolphins could communicate actual 3d maps or sonographic images using sound, rather than just describe them.
A universal language can express anything that can be expressed in language. If qualia are outside the realm of language, that wouldn't make English less universal.
You may say dolphins use a language, and then feel qualia in response. Similarly, we can imagine a human who is genetically engineered for his skin to pinch when he hears certain sounds, so that he would feel the pinching quale. I think in neither case should triggering hardware features by sound be considered language. A nice example is "clap on" devices -- surely that's not language.
This is sort of like the difference between playing a text game like Zork or a 3D MMO like World of Warcraft. English only descriptions leave the details up to your imagination. 3d graphic imagery communicates the actual picture.
Further, really detailed and well written prose can describe 3d scenes -- and help the reader visualize them -- better than everyday photographs, 3d computers renderings, what dolphins have, etc... There is no limit to how much detail prose can include.
As a practical matter, we do not use English to its full capacity, and it's inefficient for some things, but still it is universal.
They're universal languages for people; for brains that specifically evolved to swing through trees, pick bugs out of our friends' fur, and to chase down antelopes. It's software that's pretty specific to the hardware.
What if we just don't have enough in common with a critter that evolved in a fully 3D environment with no hands?
English can also, of course, incorporate 1s and 0s and thus express anything that could be put into a computer (including a dolphin's brain, which is a computer). This would be horribly inelegant and slow, and I don't think it would ever come to that, but English is capable of doing it.
Just like a universal computer is quite a simple and minimal thing, so is a universal language. This is a common theme about universality: as soon as you start to get a reasonable amount of generality in a system, you often find you've reached universality even if you didn't intend to.
Universality will be discussed further in David Deutsch's upcoming book _The Beginning of Infinity_ which I highly recommend. The book also addresses the particular issue of the circumstances of evolution of our brain and whether that fundamentally limits us.
But humans can communicate 3d scenes and imagery in words. Perhaps inelegantly. But universal turing machines do lots of things inelegantly and it doesn't really matter. English is universal language (taking into account its ability to define new words), so we're not missing out on anything fundamentally important (and nor are, e.g., the French missing out on anything fundamental by not speaking English: French is universal too).