As a musician, all I know is that if you call the SOUND command from BASIC, it behaves differently regarding volume on the CPC464 and CPC6128.

On the CPC6128, volume goes from 0 to 15. 0 is mute, and 15 is the loudest.

On the CPC464, volume goes from 0 to 7. 0 is mute, and 7 is the loudest. Then again from 8 to 15 where 8 is mute and 15 is loudest. So 0-7 is the same as 8-15.

I noticed this rather early because I did a lot of music in BASIC as a kid.

This means that if volume 10 is used, it'll be much lower on a CPC464 than on the CPC6128.

I don't know why it's like this. My best guess is a design flaw on the CPC464.

Don't know what it's like on the CPC664.