I remember using a 4x drive to burn my audio cd. When burnt at max speed, there'll be distortion (expensive lesson, lots of errors, lots of wasted CDs). So I had to burn at minimum speed to keep errors to a minimum.
Those days, professionals would use CD Architect.
These days, cd burners have data buffers to prevent errors. I use Nero lite version, sometimes the free ones of Windows Media Player and Realplayer, and the quality is just fine.
Still, it's best to avoid burning important audio cds at max speed. 8x should be quite safe.