Thanks. I missed the electric part. Although, I can't think of instruments that are electric besides the guitar that could even be put in the game.
They could easily add an electric bass; a string bass and regular bass are not very different and a shift between them is all about getting used to it, and there actually is an electric string bass. There are also electronic drums, which are slightly different than acoustic drums, but they fit perfectly for the Sims; you simply add an amplifier at live gigs, and that's it, really. My point was that the electric guitar doesn't sound right with all the acoustic instruments while jamming, so I'm forced to use an acoustic guitar.
However, the cheap Late Night piano is implied to be electric, due to it being a keyboard.
There isn't really much to do with the keyboard since you can't make different sounds with it, and musicians that use electric instruments can use either pianos, as the sounds aren't completely apart.
Which brings me to another part that I haven't though about - lack of pedals for the guitar. But that's just too much micromanagement... even though they could just edit the window (like when you're working, you can choose your method of working), and based on the pedals in the room or whatever. So yea, it can work as well. The music in the Sims is too undeveloped, really.
Bass.
I'm upset that there's no brass instruments though, in addition to the lack of a bass.
There is, but it's only a stringed one. I can see why they did a stringed instead of a regular, since it would look more or less like a guitar (the animation would have to different though). And yes, I'd like some more brass instruments myself.