My sim receivced the career object , which is that cool guitar but when she plays it, you cant hear any sound coming from it. I checked the sound settings and its fine. I can hear the piano but i cant hear the guitar. Has this happened to anyone?

Sometimes I have noticed that if a townie barges in (or any Sim, for that matter) and starts playing the bass, guitar, or drums, I cannot hear any other sound effects in the game except the Sim's voices. This could just be a problem with my sound capabilities on my computer, but it could also be part of the game.

Are there other instruments being played at the same time?

nope. I really want to hear the stupid guitar now, lol.