You seem to understand this concept very well! To answer your question, you would say: I like violins. I play the violin. I like drums. I play a drum. Unless the drum is a set of drums.... then you say: "I play the drums". I like trumpets. I play the trumpet. Good luck!

All of them are countable nouns. You may say: I like violin, trumpet and drum.

You could say, "I like trumpets," but that sounds a little childish. Most people would say, "I like the trumpet." Here, "the trumpet" isn't referring to a specific trumpet, but rather it's referring to a specific (type of) musical instrument (the trumpet) as distinguished from other musical instruments, such as harps and guitars. The same thing goes for violins and drums. Note as CJ Zwaan points out, for drums, you can also say "the drums" as it usually refers to drum sets.