In Processing.js projects the only sounds you can use are the ones provided. You are also allowed to manipulate those predefined sounds to change their volume and stuff. In the HTML environment you are also allowed to create sounds using the web audio API oscillators. Using these oscillators you can create basic notes. If you apply the right effects to these notes you can get it to sound like real instruments. Fortunately you do not have to take the time to do so because libraries have also been made to play music notes using sine oscillators. One such library is webaudio-tinysynth.js