Thanks Matt.
After I posted, I change the string to this and it worked.
[Media Type]=[audio] ~mix=1000,20%,{[Rating]=5},25%,{[Rating]=4},30%,{[Rating]=3},25%,{[Rating]=0} ~nodup=[Artist],[Name]
Think I'm at latest version. This likely had nothing to do with fixing it, but works now so thanks!
I'm admittally poor at these strings. I'm not even sure where strings are defined Is there a particular scripting language that define many of them? I'm no programmer but do like to play pretend!
Regards