Electronic keyboards use prerecorded sounds. I would count them as instruments.
I would say it depends on how you’re using it. In most cases I would say it’s an art form, but not a musical instrument. Of course, I also say most rap is not music but instead is vocal dance. I would say that to be making music you need to be in control of and creating melody and rhythm. Harmony/dischord is an additional but not required element. I personally don’t think rhythm without melody is music even if there are tones involved. I would be hard pressed to define melody with scientific precision though.
In most cases people using turntables do not create their own melody. They do frequently create a rhythm. So, I would put them in the same group as drums. Also, just because it is possible to use an instrument to create music, that doesn’t mean you are actually using it in that way. I could use a guitar to create rhythm, or melody, or both. It’s only music if I have both. (I’m not sure if it’s possible to have melody with no rhythm at all. I’ll have to think about that.)
Note that one person could be creating melody and another could be creating rhythm. If they are cooperating, they might together be making music. That would make their tools musical instruments. Thus drums can be musical instruments.
Blending other people’s music together in an especially moving sequence, is something else altogether. You are creating a kind of uber-rhthym perhaps. And even a kind of uber-melody when it’s done best. It’s like creating a collage of other works. You can just put a bunch of pieces of wonderful art together in a generally esthetic way and depend upon the works themselves to make it meaningful. You can also take pieces of art, and it is almost irrelevant whether they are good or not, and group them together to create an independantly wonderful work of art. So, you could feasably be making music with turntables, I just haven’t ever heard it done. I think you’d need lots of turntables and very fast hands unless you were creating very slow music.
Let me reiterate that I think that rap, dance, etc. are legitimate and enjoyable art forms, I just don’t think they are music in most cases.
I don’t know how many people would agree with my definition of music here. It’s a useful distinction for me. Unfortunately, the word music has a lot of very positive connotations and people who create wonderful art are insulted when I tell them it’s not music. It doesn’t have to be music to be wonderful.
Websters says that music is merely a pleasant grouping of sounds. That could be the sound of the crowds cheering when the Ravens defense scores another touchdown. I don’t describe that as music except in a metaphorical sense.