@hitsware - a little bit of context is always helpful 
Hey there @fredguth 
welcome to the forum!
There are several ways you can create the sounds of more ‘traditional’ classical instruments for example.
One way is to make use of samples as hitsware has pointed towards - you can use collections of samples and play these as notes for a particular instrument. There are several free libraries that folks have suggested so far - see Free Sample Libraries.... whats out there for starters.
Alternatively, you can use external hardware or software synths to create these sounds by talking to them from Sonic Pi. This is all described in chapter 11 of the tutorial: (or see https://sonic-pi.net/tutorial#section-11). There are probably examples and discussions around this throughout the forum as well.
Have a read of some of those links if they sound like what you’re after. If you have further questions, go ahead and ask. I’m sure there will be someone on in_thread that will be able to help 