Para quem se interessar, deixando de lado esses "reviews" inúteis e cheio de baboseiras de sites de informática falando sobre som, tem esses reviews de umas placas USB comentando sobre seus aspectos(levando em conta o audio propriamente dito):
Turtle Beach Micro II
http://nwavguy.blogspot.com/2011/10/turtle-beach-micro-ii.html
TECH COMMENTS: The good news is the output impedance is below 2 ohms, the noise/linearity is decent, and the frequency response and midrange distortion are semi-acceptable into a 10K load like a headphone amp. Into 32 ohm headphones, however, the distortion rises to unacceptable levels and the high frequency distortion into any load is even worse.
Asus U3:
NwAvGuy: Asus U3 DAC
TECH COMMENTS: Used to drive an amp, powered speakers, or other source with a line input, the U3 is a reasonable DAC. It has roughly double (+6 dB) the output of the similar Creative X-Fi Go when used to drive a line input or high impedance headphones and substantially more dynamic range giving it lower effective noise. This also makes it a much better choice if you want to use the PC’s software volume control with an external amp. Conversely, the X-Fi has more noise but much smoother high frequency response and is a bit cleaner above 10 Khz. If you’re trying to directly drive headphones, the high output impedance of the U3 should only be used either with headphones around 200 ohms or higher, or headphones with a very flat impedance that do not require electrical bass damping. The 23 ohm output impedance will cause problems with other headphones—especially balanced armature IEMs—and the relatively high distortion in the upper midrange and high frequencies driving lower impedances may also be audible. For most applications, especially driving an amp or line input with an analog volume control, the X-Fi is a better choice. Asus’s decision to put 22 ohm resistors in series with the output really compromised this DAC for use with most portable headphones.
X-Fi USB
NwAvGuy: X-Fi USB DAC
TECH COMMENTS: Used to drive an amp like the O2, powered speakers, or other gear with a line input, the X-Fi Go! is a decent DAC with the exception of mediocre signal-to-noise numbers. If you leave the software volume at maximum and control the volume from the amp/speakers/etc. it should have enough dynamic range for most applications. I put it slightly ahead of the Behringer UCA202 in most regards. The maximum output is a bit limited but most amps should still have enough gain. I would not recommend it to drive headphones under 60 ohms and especially not balanced armature IEMs. It also does not have enough output for a lot of higher impedance headphones including the popular Sennheiser HD600/650.
Syba Cmedia CM119
NwAvGuy: C-Media CM119 DAC
TECH COMMENTS: The CM119 performs similarly to the marginal Turtle Beach Micro II but with even more severe low frequency roll off into headphones and even higher overall distortion. Used to drive an amp, powered speakers, or perhaps fairly sensitive dynamic headphone of 80 ohms or more it’s marginally acceptable for a low cost DAC. But it’s best used for non-critical applications like voice chat, Skype, etc. For high quality applications driving an amplifier or powered speakers the UCA202 offers much better overall performance. Check back for more DAC reviews soon.
Fiio E10
http://nwavguy.blogspot.com/2012/01/fiio-e10-dac.html
BOTTOM LINE: The E10 has some issues but still is a clear step up from $30 to $40 products like the X-Fi Go and Asus Xonar U3. Overall, if it has enough output for your headphones and you plan to use the volume control on the E10 or a downstream amp, it’s worth considering—especially if you’re on a tight budget. But if you want to control the volume at your PC, you might be better off with a 24 bit DAC that offers more dynamic range and effective bit resolution like the HRT Music Streamer II (which requires a headphone amp like the O2) or a Centrance DACport with the low impedance output option. If you have difficult to drive headphones you may need a dedicated high output headphone amp like the O2 portable amp or upcoming ODA/ODAC.
Onboard no Macbook Air 5G
http://nwavguy.blogspot.com/2011/12/apple-macbook-air-5g.html
BOTTOM LINE: While there are some significant concerns, overall the Air mostly lived up to Apple’s reputation for better audio and outperformed typical laptop headphone outputs I’ve tested. It also has more output than most portable players including the iPod Touch and iPhone. I was disappointed the design (very likely the PC board layout—see the end of the Tech Section) is sufficiently asymmetrical to render one channel far more vulnerable to noise. I would have been much more impressed had Channel B lived up to the performance of Channel A. And the clipping behavior below 45 ohms and stability into difficult loads, are also potential red flags. But, with typical portable headphones, users are unlikely to encounter any significant problems except perhaps some mildly audible noise. For more challenging headphones, or critical listening, the Air may fall short but I don’t know of a laptop with better built-in audio at the moment. Stay tuned for that Asus Zenbook test.
"onboard vs Off"