what is the best speed to burn a cd?

You should burn your CDs as slow as your CD burner can burn them to ensure compatibility with the widest range of players out in the wild. A down & dirty rule of thumb I've found by burning and distributing demo CDs over the years is that the older the CD player, the slower the top read speed is for that CD player. I try to keep write speeds capped at x4 (x8 if pressed for time) and find that not only will it spin up and play beautifully on the latest Blu-Ray players, but plays just as well on uncle Buck's 1994 Toyota 5 Disc CD Changer!

It is also best to burn to CD +R or -R and NOT CD RW if you can avoid it. No matter which Blank CD media you choose always make sure to Finalize (close) the CD for widest compatibility.

NEVER apply stickers or labels to the CD surface unless you want said sticker, label, or even the entire CD stuck inside the player! This is especially dangerous for slot loaded CD players where the only remedy is to totally disassemble the player to remove the stuck CD/sticker/label.

I have had no experience with CDs/DVDs that have a printable surface applied when manufactured for use in inkjet printers equipped with a CD/DVD tray, so cannot comment on their ability to work reliably in slot loaded players but they do become unreadable if liquid is spilled on them or if CD/DVD is cleaned using liquid based cleaners.

I tried lightscribe technology to label my burned creations, but was not happy with the results that varied from CD to CD.

I ended up purchasing a reasonably priced Casio CD printer that 'melts' vinyl from a small cartridge to the CD face using heat transfer technology. Not only does it keep from adding thickness to the CD, but the resulting CD can be cleaned or be immersed in liquid with no adverse affects to the printing. You must use CDs with no etched-in factory writing or logos on top to use this type of disk printing.

The Taiyo Yuden brand of blank CD/DVD media gave me the best and most consistent results with less rejects than any other brand out there. Some 'name brand' media manufacturers sell rebranded Taiyo Yuden media under their own name, these can be revealed by checking the writing on the inner circle (HUB) of the media in question.

Hope this helps!

it is always a balancing act.

The slowest spin rates provide the most precise bit patterns, but ...

... that takes longer to produce a disc.

So the general answer is, 'the fastest that can still produce readable discs'.

which app are you using for burning?

are these audio files being ripped from other CDs? are they DRM protected?

which CD / DVD drive are you using with your Mac?