Hi Johannes,
Thank you for the suggestion to swap resistor R4 and R8 from 1K to 3.3K. I have just done this as it was easy to try out and this stops the headphone out level from being deafeningly loud and killing the headphones. When the Volume knob is at 10 this works fine so long as only 1 voice is playing but if I play more than 1 voice I do get distortion on the headphone output. If I turn the volume down to about 5 then I can play more than 1 voice without distortion. So would this mean that if I swapped the 3.3k resistors on R4 & R8 for higher value resistor, the headphone output will be reduced further, for example using 10k resistors, or is that too high a value for those resistors. I could try it but thought I would ask first before removing the pcb again.
Anyway thanks again, and all the best. :)