I've always used 8x for audio CDs. Recently i heard that CDs that are meant to be burnt at a high speed should be burnt that way, because the CDs are optimized for that speed and it works better than low speeds. Does this apply to audio cds as well and which are the recommended burning speeds? Is there a guide about this?

There's many variables involved, there's not really a rule of thumb. Your writer, media and player have about an equal role in how well your CDs play. I personally find that 32 and 48X (the maximum speed of the media I'm using) burns work very reliably when burned with my LG CDRW or my NEC DVDR. They seem to playback on any CD player I throw them at, including ancient ones. I think judgement on the matter should be completely binary. If the CDs play fine, you're good. If they don't, you need to revise what you're doing.

Who said that ? BTW I know the opposite... CD-R disks should never be written at their highest speed! However I think the best speed calculation is done by considering:

- Your HDD transfer rate (I had trouble writing at 52x while picking up data from my 5400 RPM hard disk because burner was fastest than the HDD and so buffer was always going to zero and then going 100%)

- The disk type (never ever think to burn a protected game at speeds higher than 8x!)

- The quality you want (lower speeds are ALWAYS better; however a "normal" data CD, 1x or 52x burned will be readable but higher speeds will generate higher writing errors and so may reduce reading speeds because of their higher number)

I think the fact that "lower speeds are better" applies especially to protected CDs and audio CDs (which I burn at 24x max).

That's not really true.
Even 4x may be too low for some CDRs or burners. They're not optimized for those speeds.

I can't make quality tests, so I use medium speeds, 8x and 16x.

An example:
http://club.cdfreaks.com/showthread.php?t=106978

Well, by burning at lets say 8X you do exclude some issues as excessive wobbling (when you burn at higher speed the disc rotates a lot faster) which may cause more "soft" read/write errors. If you want to try it out for yourself burn one disc at 8X and one at 32X or so and run Kprobe on both. It'll give you a hint.
//Danne

Well, the "fact", that the Phthalo dye is sort of "optimized" to be burnt @higher speeds,
doesn't necessarily mean, that the media made of it, cannot be successfully written @lower speeds.
Here my latest example :

Having a Yamaha I'm quiet often using its Audio Master Mode usually chosing <Best> CLV in Write Speed Settings.
The so called <Best> speed would range from 7x to 8x CLV, I guess, depending on the writer-media relationship.
To my suprise, this time the write speed proved to be only 3.5x, yet, as you can see it, the scan's still good.



Fornet 52x burnt @bestCLV