Hosting a music performance

[Francto1]

Hosting a concert where individuals play the guitar and or sing . Some have problems with audio settings where the audience can hear them but not their guitar. Seems like some have problems and others not even though they are using the same settings. Wondering if anyone has a recommendation for audio settings to resolve this issue. 

[Sharon57]

I have a virtual piano student and am having the same problem. I can hear his voice, but not the piano. What should I do?