Diesel gensets don't like to run at light loads for a longer time. A fenomenon called 'Wet Stacking' can occur, which can lead to a lot of problems.
Sound is a very dynamic load, so you should use a ballast load, like some tungsten lights or HIT moving lights, to keep a minimum constant load above 25-30%. I use 1/4 to 1/3 RMS for the soundsystem. So about 1/3 Pmax(genset) for both loads. You also have to consider the powerfactor, which will be 1 with resistive loads, but can be as low as 0.5 with other loads. I like to use a powerfactor of 0.75 for soundsystems.
Gensets are rated in kVA. kVA=Watts*powerfactor.
When you do the math the genset should be the RMS power of the soundsystem devided by the powerfactor, which leads to kVA(genset)=RMS(soundsystem)*1.33.
So a 20kW(rms) soundsystem would need a 27kVA genset, with a 8kW ballast. This leaves you at least 1/3 headroom. Don't forget to add other loads.