Just a little fun I've had thinking about the matter recently.
What do languages look and sound like in your settings?
Came up with the following:
Sylvan
Spoken - Patterns of musical notes .
Written - Pictograms, tending towards curls, whorls and spirals. Think Japanese.
Celestial
Spoken - Harmonius and formal, sorta like a blend of Latin and Tamil.
Written - Sorta Arabic-like. Long and flowing.
Draconic
Spoken - Hisses, scrapes and clicks.
Written - Arcs, and dots, as easily made by claws on stone.
Infernal
Spoken - Sorta German-like in tone.
Written - Pictograms, tending towards angular shapes and a lot of words that look very similar to other words.
Elven
Spoken - French
Written - Chinese
Dwarven
Spoken - German-Scottish crossbreed
Written - Runic script.
Gnome
Spoken - Italian
Written - Runic script.
Halfling
Spoken - Spanish-Portuguese
Written - Spanish
Common
Spoken - Pidgin English(with loan words from everything)
Written - English
Auran
Spoken - Whistles, going by pitch and length. In a pinch a flute or whistle can be used.
Written - Sentences start with a triangle, and a line emerging from the pointy end. The angle and duration that the line sticks with a given angle determines meaning. Sentences terminate on the flat side of another triangle, which points out the direction of the next sentence. Best written in three dimensions, as 2d loses a lot of nuances.
Aquan
Spoken - Bubbles/gurgles/splashes/jets of water. The number and size/volume detemines the meaning. First and last bubble define the 'baseline' volume. In a pinch, any bass instrument could do.
Written - Rings, concentric and intersecting. The degree and number of overlaps determines meaning.
Terran
Spoken - Morse, with a lot of heavy thumping. Can be 'spoken' reasonably well with a hammer, drum or any percussion tool..
Written - Shape based, with angles, depth and facets used to express meaning.
Ignan
Spoken - Light, color and heat modulation. Can be 'spoken' with a torchlight and a shutter, but would do better to use a good oil lamp and colored filters.
Written - Color and brightness determines meaning.
So, how do you see languages?
For Abyssal...you could probably have a set of meanings, then those are stringed together however the hell the user likes to make a message. A "spoken" form would be popping sounds, which are easy to make for anything in any form, even an amorphous thing, or even colors. Most chaos-related critters are shapeshifters, after all. The color route also instantly doubles as a writing medium if a mortal wants to write it down (anything made of chaos wouldn't bother).
The elemental languages are easy enough. Air elementals vocalize by whistling, wooshing, and wheezing, earth elementals through morse-esque clicking sounds (easy to make by hitting something solid with something solid), water elementals with gurgles, bubbling, and sloshing, fire elementals with crackling sounds.
Druidic... maybe it doesn't even look like a language, but a landscape drawing. Druids label the owner of a grove through feng shui. 
Halfling: either derived from Common, heavily influenced by Common, or heavily influnced Common; it uses Common script and shares a number of words.
Gnome: based on Dwarven, but more freeform grammar and with more loanwords. While Dwarven and Gnome script are similar, it's like comparing block capitals and cursive (though they're a bit closer than that).
Well, I was thinking Fire wouldn't have a strictly Spoken form(since they'd probably use light and heat modulation), but anyone with a light source and a shutter could probably 'speak' it with a horrendous accent.
And here I go thinking Halfling could be some Portuguese and Spanish mix.
And here I go thinking Halfling could be some Portuguese and Spanish mix.
Not necessarily mutually exclusive with influencing and being influenced by Common. They could be influenced by the same root languages, like how Spanish and Portuguese are derived from Latin and a good number of english words have Latin roots as well.
But yes, Spanish Halflings for the win.
And here I go thinking Halfling could be some Portuguese and Spanish mix.
There's a reason Eberron gave halflings the Mark of Hospitality. Halflings are generally friendly and have a lot of traders, so it would make sense as a significant root. I could swear I'd seen something like this before.
EDIT:
But yes, Spanish Halflings for the win.
Nah, Spanish gnomes. Gnomes get the blade bravo class, so D&D Zorro is a gnome.
But yes, Spanish Halflings for the win.
Nah, Spanish gnomes. Gnomes get the blade bravo class, so D&D Zorro is a gnome.
No, gnomes are small and funny inventors. They're clearly Italian.
Infernal
Spoken - Sorta German-like in tone.
Written - Pictograms, tending towards angular shapes and a lot of words that look very similar to other words.
FC2 actually brought this topic up before. According to it, Infernal is written in a form similar to Morse Code+Braile, but without the dots.
No, gnomes are small and funny inventors. They're clearly Italian.
This is now forever canon in my Eberron games.
Well, Gnomes use the dwarven script, so you get some atrocious German-Spanish hybrid.
EDIT:
All hail Italian gnomes.
IIRC Spell weavers speak in a kind of semaphore and their written language is based on arm positions.
Also, I get the feeling that if there was an anime based on Eberron we'd get:
Draconic: German. Because anime spells are always in German.
Elven: Chinese. There are plenty of elves and half-elves, and they have both Arrogant Kung Fu Guys and a court of divine beings.
Giant (or Riedran): English. Exotic and powerful, but somewhat crude. Riedran visitors are pretty cool, but they're trying to subvert local culture.
Goblin: Antiquated Japanese. Because they were the first inhabitants of Khorvaire and had samurai.
Orc: Ainu. Because ancient and respect for nature.
some atrocious German-Spanish hybrid
Also known as... Italian.
(Okay, to be fair it's more on the Spanish side.)
I haven't put this much thought into it, honestly. I remember one campaign where I tried to give each sub-race a language and each continent a separate Common. It turned out to be a big pain in the ass and a pointless skill point sink.
That flavor is cool though. I have been known to try to adopt voices for NPCs, so I guess I do put some thought into how things sound... just when I'm speaking English, though. 
I always had my goblins speak Spanish. Common-speaking goblins tended to sound like Cheech Marin.
Running gag at my table is that Undercommon should Ebonics. "Fo' shizzle my Drizzit!"
Aquan: Bit of an interesting case. The elemental plane of water is, well, nothing but water. Aquan isn't sound-based because everything is drowned out by the constant roar of noises propagating in every direction. Instead, it's based on the pressure made by the water against your skin. There's no fixed phrase that means something because a speaker has vary what he does to account for currents, other creatures, local architecture, whatever. In the open air, you speak it exactly the same (taking into account your surroundings) and a listener has to determine what sort of pressure patterns he'd feel if he can't feel them normally. The script is a stylized representation of those pressure patterns, which makes it a lot easier to read Aquan than to speak it if you're not an aquatic race, since a written Aquan phrase isn't context-dependent.
Ignan: It's fire-based, obviously, but since the entire freaking plane is on fire, temperature fluctuations distinguish what you're saying from random background noise. You could seriously have the script look like a graph of temperature over time and just be a single line spiking up and down, with breaks indicating words or something.
^^
Aquan's an interesting thought, I went with pulsed bubblings because thats pretty much the reason, you need to make it through the cacophony. Good call on the context dependency though.
Ignan, I updated the OP with something. Color and temperature both apply, largely because color propagates further than variations of heat when well...the whole place is on fire.
