Do you want them to turn on full or gradually brighten? I'd think that on full (while its still light out) then gradually dim as it gets dark, then possibly twinkle would be neat. Easy to do using the PWM Outputs.

Have a look at...

http://tim.cexx.org/?page_id=374

and about halfway down the page is a schematic and description of an "Analog beat detector".

This circuit should do what you want. The comparator output is an impulse detector that tracks along with the background sound level. The lower Diode/C/R network at the "-" input is the "reference" or "sound level average" network.

Approx. what is the fan tach frequency? It's often a fraction of the fan RPM. You may be able to use the  millis() or micros() to measure the delay between tach pulses (period)  and then convert the period to frequency (f = 1/t) . An RC network (integrator) on each fan tach and analogRead() to measure the output voltage from the network would work, which essentially converts the pulse rate to a voltage and won't be terribly accurate but should be good enough for what you want.

"...sound abnormally exceeds that sound level" and "sensor detects a loud noise" are somewhat different. The former requires an sound level average while the latter, peak detection.

Do you want one (which one) or both? The Arduino can do both in software (with a little hardware to create an amplitude envelope, a "DC" level that reflects the sound amplitude) or both can be done in hardware and the Arduino can simply read the two levels.

The brick includes a "high sensible microphone", but no amplifier, which isn't very "sensible".

The 317 will work as a current source but is not directly PWM-switchable without additional components. This thread discusses some options...

http://arduino.cc/forum/index.php/topic,102101.0.html

I think the circuit with the transistor shutting off the ADJ pin will allow for a low-power transistor since it's only gating the high current output of the regulator. Uses a few more components but looks to be a better design (unless you're building a lot of them and looking for lowest parts count). THe other circuit will have large currents through the transistor and need a beefier heat-sinked transistor.

There are lots of devices out there, particularly logic-level MOSFETS, that will directly take Arduino PWM outputs and drive high-power LEDs.

Although I haven't used this one, I may in the future...
http://www.onsemi.com/pub_link/Collateral/CAT4101-D.PDF

THis implies that you have soldering skills though. Otherwise there are many LED drivers out there. Just be certain to choose one with "logic-level" inputs, meaning that it will work directly with the TTL PWM signals from Arduino and not need level-shifting interface circuitry.

> You need to use a PNP transistor, not an NPN.

Can you explain why just the NPN alone wouldn't work?

And the data sheet states "Please DO NOT connect "DIM-" to "-Vin".

Where else does it get connected?

Keep in mind that brightness levels of 0, 25, 50, 75 and 100% will look to the eye like brightness levels of about 0, 60, 80, 95 and 100%. This is due to the eye's nonlinear response to light. If you want visual levels of 0, 25, 50, 75 and 100% then you may want to start with 0, 3, 20, 50, and 100% and tweak from there. I recall seeing some algorithms that will calculate the relative "eye" brightness from LED % brightness. 

Where does the diode go?

By "in series" I assume you mean with the comparator output into the digital input.

I found (and have a few) comparators with TTL outputs which should work.

I have an opamp as a comparator whose output I want to read with a digital or analog input pin and trigger an action when the comparator output goes high and a different action when low. Currently using +9v/-9v supplies for the opamp and want the comparator output to be compatible with the analog/digital pin (0 - 5v). A 5.1v zener from the comparator output to Gnd clamps the high output to around 5.1v but the low level is -0.7v. Is this -0.7v bad for an analog/digital input and how could I better the circuit to more accurately yield 0-5v comparator output for the analog/digital input?

You can also set up an elapsed timer of sorts using millis() or micros() and save the time T0, then periodically check the time in a loop that goes off and does something else, and when the periodically checked time is greater than some chosen time span after time T0, trigger the other event. Kinda like watching your stopwatch until so many seconds have passed before doing something else. This allows you to "parallel process" in a sense based on elapsed time.

> 14 digital + 6 analog = 20

The 6 analog pins are input, not output. I did miss the RGB part though so, with the 14 pins can only light 4 RGB LEDs without adding multiplexers or drivers.

20 LEDs will somewhat of a challenge with only 14 outputs, won't it?